Oona's Grace generally doesn't appear to be that difficult to play either. When you have relevant spells (that you don't have a reason to hold), you're going to play them. If you have the time to, you're going to retrace your Oona's Grace.
Pretty straightforward, right?
Retrace cards are really hard to play correctly. Sure, Oona's Grace and Call the Skybreaker might appear to be easy to use. But if you go on autopilot and fail to recognize how the game state is changing, or what your other options are, you might end up burying yourself.
Just having a single retrace card in your deck should significantly change the way you play the game, especially if the game goes long. Whenever you have a retrace card in your deck you have to ask yourself, "How many lands do I need in play?" And unfortunately, you can't get away with just asking yourself that question once.
Sure you might give your deck a look-over and say "Oh, I don't have any six-drops. I guess I only need five lands in play. That means that I can stop playing lands after I have four in play, and hold the fifth land (and beyond) until I need to play a five-drop." And while that might appear to be an easy solution to your problem, it might cause you to miss out on a lot of good opportunities.
For example, the other day I had drafted a very aggressive mono-red deck with a Flame Jab. The most expensive spell in my deck was a Kulrath Knight, which costs five mana. Armed with this knowledge, I knew that I would want to stop playing lands after my fourth land drop unless I had a good reason to.
CUE good reason to play more lands.
While this was a fairly obvious play given the context of the situation, the important thing to note is that you always want to be aware of your options. Even if there is no single spell that you need a lot of lands to play, if you get into a spot where you know you are going to want to play two spells in a single turn, then you'd better put yourself into a situation where you will be able to do it.
Time to Retrace your Steps
Are you going to use your Flame Jab two or three times in a turn to deal with pesky creatures? Are you going to use it to enable your small creatures to attack into bigger creatures, leaving your opponent with a massive headache as he or she tries to figure out safe blocks? Do you hold your lands waiting to unload a string of Flame Jabs for 5 or 6 damage straight to your opponent's face?
Any of these options can be quite good, you just have to figure out which one (or two, or three) of them is appropriate for the game that you are playing.
A lot of the time you will find that you are going to use Flame Jab for a variety of different things in a single game. Often when I have Flame Jab in an aggressive deck I'll use it to knock off a key blocker or two early, and then once my opponent stabilizes the board I'll start stockpiling lands that I will eventually send straight at their noggin.
Retrace the Troops
In fact, this is a problem that occurs whenever you have a card that allows you to constantly pump out creatures tokens. Whenever this happens, you have to answer the question: how should I value the tokens?
There are plays that you are almost always happy to make, such as trading your tokens one for one against physical pieces of cardboard. Generally, when you are trading fractions of a card for an entire card, then you are getting the better end of the deal. (See below for more on marginal card advantage.)
Then there are the plays that you usually don't want to make, but sometimes feel that you have to, such as attempting to trade four of your 1/1 tokens for a green 4/4.
Then there are the plays where you just don't know what to do. Do you want to trade two 1/1s for a 2/2? If it's early in the game and you're clearly on the defensive or the offensive, then you will probably want to make this exchange (though admittedly, there won't be too many times when you're on the offensive that you'll get the chance to trade two of your 1/1s for a 2/2).
Let's say that it's later in the game. You are attacking with an evasion creature or two and your opponent has some large creatures (or is about to play some large creatures), and is attacking with a 2/2. Well, you're going to have a tough decision to make.
Do you want to trade two of your 1/1 tokens for the 2/2, knowing that you will still be able to pump out more tokens that can chump-block future attacks from giant monsters? Or do you want to save your tokens, knowing that you will get a lot of utility out of them later, figuring that it won't be long before you draw a creature that will shut down your opponent's lowly 2/2 with ease.
Retrace Cards to Stay Away From
While Waves of Aggression might at least have some application in a hyper-aggressive creature deck or in a deck with a lot of Battlefield Mimics, Worm Harvest should almost never see the light of day.
While the incrementally growing token generator might seem good, you have to keep in mind how much time and effort you are going to have to put into getting the game to a point where you are going to be getting value out of the five-mana sorcery. Unless you are playing the most grindingly painful control-on-control matchup, stay away from this card.
This card is awesome.
Being able to make a copy of the best creature on the board on a regular basis makes it pretty hard to lose unless you are already way behind. Even if you are way behind, Spitting Image can be a great way to get you back into the game.
Well, six mana is very different from seven, and that difference has never been more true than it is here. You literally get an entire extra premium creature out of Spitting Image (as compared to Call the Skybreaker) when you draw it fairly early.
This is about as big of a difference as you can get between two cards that have similar effects upon resolution.
You should always be aware of the difference between a card and a similar card of a higher mana cost, as it will often mean that you are losing at least one full use of the card.
Choosing Which Retrace Spell to Play
A lot of times when you are choosing between playing two cards you know that you will be able to play the other one later. Then there are times when you have to choose between discarding one of two cards, knowing that you will (probably) never get a chance to use it once it's gone. Then there are times when you have to choose which of your creatures you're going to block with, knowing that once it's gone, it's gone.
We have to deal with these types of questions all the time.
But what happens when you have to choose between two playing two different retrace cards? This type of question, while completely unique to Eventide, has countless parallels that you can compare it to.
Choosing between retracing a Cenn's Enlistment or an Oona's Grace is a question of whether you'd rather have two 1/1 creatures or a new card. This is exactly the type of question that people have been asking themselves for years. And to be perfectly honest with you, this is exactly the type of question that keeps me glued to a game that I've been playing since I was a small child.
The source of your effects doesn't always matter so much as what those effects mean. Sure, you have to pay close attention to what your future outputs are going to be, but if you need two 1/1s more than you need a new card, then it often won't matter where they're coming from.
While it doesn't necessarily matter where your effects are coming from, you always have to keep in mind what you are giving up to get your effects. If you're retracing your Cenn's Enlistment to chump-block instead of retracing Oona's Grace to draw something relevant, in a game where you're not getting in for damage, then you're either in a pretty bad place or you're doing something wrong.
The lessons that can be learned from retrace are definitely lessons that you can carry over into future formats. Retrace cards are reusable effects, and while we don't see these types of constantly reusable effects on instants and sorceries that often, we do see them on permanents all the time. It doesn't matter if you're generating your tokens with a Cenn's Enlistment, a Selesnya Evangel, a Sprout Swarm, a Squirrel Nest, or a Thallid Germinator. They're still creatures that you need to figure out what to do with.
The same principles apply to Flame Jab; you could just as easily have a Lava Dart or a Siege-Gang Commander that you need to figure out what to do with. Whenever you have a multi-part resource, you're going to have to pay a lot of attention to see exactly what you need to do with its pieces.
Whenever you learn something significant about Limited, you are going to want to carry the principle behind it into the future. Sure, the exact same principles might not apply in future formats or future games, but if you are thinking about your problems in the right way, good things tend to happen.