Couple Points first:
A) Because of Restore Life & Limb, level-draining seems to be far more dangerous than anything that can just outright kill you.
B) Some low level monsters, like the Wight, can level drain you. This creature is worth 80XP. It can potentially take away hundreds of thousands.
C) Drained levels cannot be removed, short of ritual magic or wishes.
I like that there's something dangerous that can actually hurt or even kill a PC that's not easily fixed (most PCs above 3rd level can easily afford Restore Life & Limb).
However, I wonder if the XP value of creatures with such abilities is poorly attributed.
Shouldn't something so dangerous to low and high level characters deliver more XP? Special abilities aren't finely tuned for their "power level" in the game. A Giant Leech with a special ability to drain some blood each round does not really compare to losing a level or two.
I don't know if I'd want to go through and give each specific ability and XP value, but I'm mostly just wondering if it was ever considered to increase the XP value for creatures that level drain (or have similar devastating abilities)?