Though if you just develop for one I suspect it makes things a world easier than the computers which vary so much in specs.
Not necessarily, Wii and PS3 are VERY difficult to develop for, and PCs allow much more flexibility and choice than the 360.
I prefer new games to come out of the 360, because I always look at them and my PC barely makes the Minimum requirements, so with the 360 I can be pretty sure
1) The quality will be high
2) I'll be able to run it without it jittering around like !@#$%.
With a PC game errors can be caused by anything from Game code to your graphics card.
With the 360 the only real errors (ignoring the red ring of death I've only seen when I accidentally blocked the fan) I've seen are caused by code glitches.