By making it a money equivalent you have a way of easily balancing your characters against comparable D&D characters. Characters in D&D are truly built using two kinds of points, XP and GP but in a lot of settings we don't want to actually use GP so I think it's best to simply have a GP surrogate.
For one thing, your players will like you better since they want to hoard their XP for more levels and they know they're already getting the shaft by playing in one of those awful Star Wars-like games where you don't get any pay-off for missions.
Secondly, a more subtle point is that challenge ratings, modules, monsters, and even class abilities are all designed around the assumption that players will have certain amounts of magic items at certain levels. By funnelling off XP to pay for those abilities you break the connection between experience level and magic item power level. This is because you could wind up with a 5th level character with a boatload of magic item style powers surrounded by 10th level characters with more reasonable "magic item" sets.
From a purely gaming perspective I also think it's just a more fun idea to have two or more kinds of resources. In computer games like Warcraft you have Gold, Wood and Mana (I think; it's been a while), in V:tM you have xp and Bloodline rank, in Ars Magica you have skill points and Vis. I'm not sure why but I've generally enjoyed games where I have to balance off a couple resources (but not too many, 2 to 4 generally seems to be fun without being cumbersome).
I'll insert a shameless plug here too; I suggest a few options and specific implementations of this stuff in Bodies and Souls: 20 Templates, on sale at RPGNow for 5 bucks.
I certainly don't think you need to go the GP route. But I think doing it right makes it a lot easier to import stuff from other d20 supplements (like running the occasional fantasy module, drawing down some stuff from Dragonstar, or just pulling some NPC writeups off the net). It also makes it easier to take the work you did on Dark Angel rules and port them back over to a fantasy d20 game, giving it a lot of re-usability value.