This isn't a case of "what to do", but rather "what it means". Delricho was responding to an assertion that "XP is in no way a measure of lifeforce", with rules observations that imply exactly that (OD&D-3E).
A person can reasonably express dislike for it, or a proposal to change it. But when someone says that it simply isn't such a thing, then historically for D&D, they are wrong.
Nowhere in the mentioned post by justanobody, did he state or imply that this has never been a part of any edition of D&D. I'm pretty sure he was simply talking about what a few of us on this thread have been saying, that for us, the abstraction of "Experience" doesn't make sense when applied to "Lifeforce" - whether it is or isn't supported by, or stated in, the rules of any D&D edition - and
not that it
isn't, or never has been,
historically, a part of D&D. I think trying to say he intended exactly that is just looking for any reason, no matter how small, to pick a fight. However...
I really don't remember how "Experience" was defined in 2E (and I don't have my books available), and I can't speak for earlier editions (didn't play them), but from the standpoint of the 3.5E PHB and DMG, I'd say although not entirely wrong, Delricho's not entirely right either. This is because 3.5E's definition of experience is contridictory (and although I'm not sure, probably holds true for past editions also).
3.5E PHB, pg. 58
"Experience points (XP) measure how much your character has learned and how much he or she has grown in personal power. Your character earns XP by defeating monsters and other opponents. The DM assigns XP to the characters at the end of each adventure based on what they have accomplished. Characters accumulate XP from one adventure to another. When a character earns enough XP, he or she attains a new character level (see table ..."
3.5E DMG, pg. 36:
"Experience points are a measure of accomplishment. They represent training and learning by doing, and they illustrate the fact that, in fantasy, the more experienced a character is, the more power he or she possesses. Experience points allow a character to gain levels. Gaining levels heightens the fun and excitement."
- These paragraphs imply that Experience Points are a measure of aquired knowledge and experience gained through accomplishment, and that this KNOWLEDGE and EXPERIENCE increases the power of the character (represented by levels). The best definitions of power, within the context of this paragraph, would be skill, knowledge ("Knowledge is Power"), or even "Prestige". This is because this applies to all characters, NOT JUST SPELLCASTERS. Power in this context does not necessarily imply lifeforce except in a VERY loose interpretation. That only occurs in the next paragraph.
"Experience points can be spent by spellcasters to power some of their most potent spells. Experience points also represent the personal puissance that a character must imbue an object with in order to create a magic item."
- This paragraph is essentially saying that Experience Points (and by extension, levels) is an abstract idea that means multiple things at once (as a lot of things do in D&D). Puissance can mean lifeforce, and I'm sure is intended that way in this paragraph.
The problem arises because combining lifeforce and aquired knowledge together, under one abstract construct, is like saying education and spiritual growth are the same thing. It's a combined abstraction that doesn't make logical sense to many (including myself), thus the reason for the question posited by the OP in the first place.
Hit Points: (3.5E PHB, pg. 136)
"Your hit points tell you
(sic - define and quantify) how much punishment you can take before dropping. Your hit points are based on your class and level, and your Constitution modifier applies."
- This isn't a perfect fit for lifeforce either, although I'd say this is just as much an abstraction as XP is.
Constitution: (3.5E PHB, pg. 9)
"Constitution represents your character's health and stamina. A constitution bonus increases a character's hit points, so the ability is important for all classes."
- Also not a perfect fit either.
So, we are left with trying to make a logical deduction of what losing lifeforce would mean and how it would affect a character.
- Would a loss of lifeforce be expected to physically affect a character? - I think that would be a logical outcome, therefore probably detrimentally affecting the ability to execute physical actions (skill checks, attacks, resisting damage, etc.).
- Would a loss of lifeforce be expected to affect the spirit (Ki, energy, soul, etc.) of a character? - I think this would also be a logical outcome, best modeled with a reduction in willful resistance (will saves), and reduction in ability to expend or channel energy (whether arcane or divine).
- Would a loss of lifeforce be expected to decrease or eliminate the acquired experience, knowledge and accomlishments of a character? - This is where it no longer makes sense to me, making a decrease in XP a hard thing to swallow.
So, from this prespective, a reduction (whether temporary or permanent) in ability to cast magic and accomplish physical feats makes more sense, especially when it's reinforced by most narrative fiction (from which most of the concepts of D&D are taken). Just because it's not the way it's always been done, wether RAW or not, doesn't make this a bad idea. Restricting the definition of Experience to aquired knowledge and accomplishment seems to make better logical sense. This also makes a lot more sense to me for the draining abilities of undead. To me, the mechanics should be one and the same.
When Jonathan Harker is drained by the Brides of Dracula, does he lose his knowledge, experience, and accomplishments? No. He is permanently affected physically (weakened, gray hair) and mentally (sanity, not knowledge). In fact, afterwards one could even make the argument that his Will was even stronger now that it was focused on Dracula's destruction. His memories remain intact, and his knowledge, garnered through experience, is unaffected.