Yep. Sure it has a to hit roll, but it doesn't do damage. If I use harm on a commoner with say 5 hit points, I drain all but 1d4 hit points. If I use it against that epic level barbarian with 500 hit points, I drain all but 1d4. I don't roll dice to see how much damage I do, I roll to see how much they have left. If it was a spell that delt damage, then I'd be rolling to see how much damage I do.
I think that's his point, small, blue, and hyper. If harm inflicts damage, it should inflict double damage on a confirmed critical hit. Which would be ...
a) reduce the opponent to 1d2 hit points. ( )
b) reduce the opponents hit points by twice his current hit points, minus 2d4. ( )
c) um, I dunno ...
And, today's final D&D trivia question: Name the high-level combat spell which is completely useless against an opponent with 1 hit point. You have 30 seconds ...