With multiclass, you obviously have to figure out how to play more classes. You'll need to know how to gear them, how to macro them, things like that. I don't personally find it a big deal, but then, I've always done multiclass.

As far as min/max, it's pretty irrelevant at this point in the game. It might matter for PVP, but for 5-man PVE in a standard group, as long as you don't suffer narcolepsy and randomly pass out during MoP encounters there's nothing you can take down with a single class that you can't with multiple or vice versa. the only point where that is NOT the case that I've seen is Cata dungeons - in which case ranged had a significantly easier time of things than melee did - but that still doesn't rule out multiclass and may be irrelevant since you're not running the entire group on your own, which gives more leeway to control your toons.