I did it because I don't need 4-core power. The apps I run would rather have a 2 core 2.66GHz system than a 4 core 2.00GHz system.
Well, now I have a 2 core 2.66GHz Mac Pro for less than what it would have cost me to get a 4 core 2.0GHz machine. As for AppleCare, nothing was harmed or damaged in any way during the removal so I can throw in another 5150 and it is as good as new. Warranty intact.