I wouldn't exclude a class simply because the fluff didn't fit my setting. The point above about the separation of crunch and fluff is important. If you strip off the fluff all a class is, essentially, is a feat tree that provides additional rules. When I start with a character concept it very often isn't tied specifically to a class, so I'll treat multiclassing, or even single class choice, as a practical matter of what rules best represent the character concept I have in my head. This happens, for example, with the Bard class all the time for me - the crunch suits a lot of builds, and not all of them are performers who deal magic by keying up Toss a Coin on their medieval IPods.
I'm sure not everyone treats classes like I do, but I try not too get to caught up in the fuff, at least not at the expense of a good character concept.
I'm sure not everyone treats classes like I do, but I try not too get to caught up in the fuff, at least not at the expense of a good character concept.