I think it's a valid point, and one that the rules cover. In the 3.0 DMG, at least, they say something about situational modifiers to XP -- what is it, the orcs running out and attacking the party versus the orcs flying overhead on hang-gliders and dropping boulders?
If the party is fighting a creature that, for WHATEVER reason, can't use its abilities to its full potential, they shouldn't get full XP for taking him out unless they are the reason that he can't fight at his full potential.
Examples:
The party is trying to kill a celestial that possesses a massive area effect damaging spell. They see him in the middle of a crowded marketplace and attack, and the celestial can't use his big boom power without hitting a bunch of innocents.
Are his powers limited? Yes. Is it due to good PC tactics? No. Therefore, the celestial is worth fewer XP
The party is trying to kill a red dragon. They know that the dragon will not kill the kobolds that serve it, because, for all its selfishness and greed of normal red dragons, this red dragon views the kobolds as its children. The party pays a great deal of money to an alchemist-mage, who gives them a cask that, as they attack the dragon, opens up to fill the air with extremely flammable gas. The dragon smells it and knows that breathing fire will cause the entire place to blow up, killing all of its kobold "children". Frustrated, it refuses to use its breath weapon, and the party has an easier time of it.
Are his powers limited? Yes. Is it due to good PC tactics? Yes. Therefore, the dragon is worth the full XP award.
Sounds reasonable to me, and it's in the rules as written.
