While I generally agree that it would probably make enchanting too easy, I can think of a good reason to implement this...
Red Dragon Disciples. There's a known bug with RDD: the ability score boosts you get from RDD levels do not disappear when you lose those levels -- they modify the base ability score of the character when you gain the level. Even worse, this means you get them AGAIN when you gain those levels back. At level 20, RDD gets +2 Charisma, so an RDD enchanter who hits level 20 would be gaining +2 to Charisma all the time. Considering that RDDs come from classes that greatly benefit from high Charisma, this is a pretty big problem.
If you could gain at least some XP above level 20 to use with enchanting, while you could still exploit the bug if you really wanted, you wouldn't have to trigger this bug to enchant anything. I'm not sure if that's reason enough to add a small XP buffer above level 20, but I thought it ought to be mentioned.