Ahh, then the eariler point about sematics was correct. "Quite often" =/= "majority of time". In other words, we're saying the same thing, just phrasing it differently.
Add to that the fact that you allow monsters to wander past fighters without penalty and it does seem to show why you would think it happens so often. Note, I never actually played the "must be hit during casting", that was just me misremembering the rule. We always played it "lost if hit before casting", but, since virtually no monsters have ranged weapons, there were almost no monsters that could cast magic missile, and we didn't allow enemies to just walk past the fighter, we didn't see spell interuption very often. LIke, maybe, maybe once per session and probably closer to once per adventure.
Again, if your MU is in melee with a troll, the players have screwed up FAR more than my players ever did.