One of the things I've noticed about the perennial debates about level/energy loss due to undead or demonic attack* is people complaining about or supporting the interpretation of energy drain as loss of memory.
A 4th level thief is attacked by a wight. Its chilling touch drains the thief to 3rd level. The thief can no longer use the Read Languages ability. Is this because the thief is so badly shaken to the depths of his soul that he just feels too confused to decipher maps, or is it because he really forgot how to do that? A 5th level MU is drained to 4th level and loses 3rd level spells; is this because she can't muster the strength of will needed for spells of that power, or is it because she forgot how to cast Fireball?
I realize that some people think of levels as linked to training or education, partly because of the "class creep" that introduced new classes who gained specific abilities at specific levels, partly because of later additions like weapon and non-weapon proficiencies. But it still stuns me that, even if people want a skill system or training in class abilities, they would interpret reduction of those abilities as forgetting how to perform those tasks. There's nothing in legend, literature, or even the original books that characterizes the undead as causing forgetfulness.
You would think that people wouldn't interpret energy drain as memory drain, since this raises the question: does that MU have to re-roll the chance to add Fireball to her spell books? Does the thief have to pay the training costs for increasing a level, if you are using those rules?
* Funny also that no one complains about the AD&D rule about losing a level because of alignment change. But I'm against that interpretation of alignment, myself, so I won't dwell on it. Still, why would changing alignment make you forget anything?