d20Dwarf said:
In D&D, XP simply IS the barometer against which a character's power is gauged, not levels.
I disagree. XP are a
resource, but levels are the fundamental barometer for the game.
The appropriate encounter level for a party is based on
average party level. When the DM creates NPCs to challenge the party, those NPCs are rated by their
level, and those NPCs have items according to their wealth by
level. If an NPC has item creation feats, you
do not subtract the XP from the PC and adjust statistic accordingly... you translate that XP loss into GP loss, and the basic GP is, again, based on
level.
XP costs are primarily a means of creating a conundrum for players who wish to get benefits that do not directly spring from class and level. It does that, but it distorts the meaningfulness of levels. In moderation, this is not a big deal, but you should not, IMO, use it as a basis mechanism.
As to why, consider one of the fundamental changes between 2e and 3e: the way multi-class characters were done. Consider why this was done... because for a single class character to gain abilities, they had to progress along a non-linear XP curve, but multi-class characters effectively applied a linear multiplier to their abilities in exchange for a small non-linear hit. In these XP-based benefit schemes, you are effectively binging the 2e advancement scheme into 3e.
In short, I think that using XP costs for spells and magic items is a poor excuse for blowing the doors wide open on using XP expediature as a major resource for character abilities beyond its role in level advancement. Yes, it does play a role in magic item creation, but AFIAC, you shouldn't use XP when you can use levels to acheive the effect you are looking for. And with prestige classes and feat chains, you can.