It would defeat MSFT's primary purpsoe with X-Box, which is to promote their APIs and thus control development.
.... and to RULE THE WORLD!! MUHAHAHAHAHHAHAHAH!!!!
On the other hand, I have heard that the X-box 360 isn't nearly as bad to develop for as some think. You don't need to use multiple threads for one and two debuging them isn't nearly as hard as the /. poster suggested it is. Several of the top games being developed for the 360 are using multiple threads. I think this is more about people fearing change. Multi-threaded programming, especially in games, is not something people are used to. Those embracing it appear to be doing fine.