The balancing element under discussion here is that wizards get attacked and lost spells often enough that it becomes a real factor in playing a wizard (or any caster).
IME, it happens so rarely that it's not a real balancing factor. Getting attacked from multiple directions by opposing forces numerous enough to actually perform that sort of manoeuvre just isn't all that common, again, IME. 3e in particular tends to favour enemy groups of 2-4. Anything larger and the individuals are just too weak to be a real threat.
Then again, I've always seen games where monsters are the norm, not humanoids. And ranged weapon wielding humanoids just aren't that common, again, IME. I mean, while spears might be pretty common, ammunition becomes an issue. You chuck your spear, well, just how many spears is your average goblin carrying? Axes and swords tend to be a lot more common. Bows and slings aren't used that often and really don't make sense for a number of humanoids - I mean, how many lizard men are going to have a bow or crossbow? Or troglodytes? Ogres aren't really known for using slings.
It's not that terribly difficult to keep the caster out of harms way usually. Particularly if the party is careful.