If you have the choice between a fire spell and a water spell, that gives you two choices. It you're given a third option (eg an air spell), then that adds complexity. Instead of having 50% chance of making the best move by accident, it's now 33%. You either have more choice and more complexity, or you have less.
Variety that comes from ignorance, is a kind of variety one would be unable to perceive.
It is fundamentally different to understand that you are unable to make the optimal play(you don't have air spells against an enemy weak to air), and to be ignorant of it(you don't know that the enemy is weak to air, even though you have the spell).
If one eliminates ignorance from the equation, which is how a game should be balanced(i.e.: assuming the players know the rules), having the optimal move available means that the player will(aside from being deliberately ignorant), use that move.
Further more, if, as LordCrash argues, we give to the player a significantly wider range of tools, the odds of a scenario where the optimal move is unclear(there are two targets weak to air) diminish significantly, as the cost of opportunity in making the optimal play is diminished by the ability to repeat it if necessary.
Essentially, as the player becomes more likely to have a clear 'best move', the challenge of combat is shifted away from decision making(when and how to use the limited toolset).
In simpler terms: Just because you have more spells, it doesn't increase the variety if you'll end up using them the same way each time.
Also, what is a limited arsenal?
What is a correct loadout?
Does having the correct loadout exclude having variety in combat?
By that I mean if I have the correct loadout will ever battle be the same?
Does having a limited loadout necessarily mean a greater variety in combat?
-One that forces a player to choose between various aspects of combat.
-One that offers a clear optimal move within a given scenario.
-Obviously not, but as part of advocating variety, there should be both encounters for which your deck works well, and ones that it doesn't(and not just the former).
-As mentioned in the previous answer, having a limited loadout makes it more likely that the player will find themselves in an encounter that they cannot resolve in the optimal way, and more likely that their unique solution will have to be different.
