I know that Reiner Knizia's games are just clever abstract's not simulations, but I do think that a good theme helps the game.
Battle Line is just a card game about building sets of cards, not all that different from Lost Cities, but the ancient battles themes works really well, making the game more approachable for the literal-minded such as myself.
That it's not a simulation is proved by the fact that the most powerful card is the "chariot," which was well past its prime as a weapon system by Alexander the Great's era.
But arranging formations of historically named cards and leaders does make it a little more real, a little easier to remember.
I know euro gamers aren't as theme-dependent as wargamers, but I think games like Battle Line demonstrate it's not unimportant, after all.