Blu-Ray adoption is still low, especially given the premium you have to pay for Blu-Ray discs, thats likely the reason for it not being on the Xbox.
However adding Blu-ray would certainly increase its adoption rate which would be a good thing , since it would help to lower prices. But either Microsoft don't want to help Sony, or they have a new product in the wings to supersede the Xbox 360 (which hasn't been exactly trouble free), other than that i can see any other reason for not providing it as an add-on.
Looks like i'll probably be getting a PS3 for my console needs with this news.