There's no reason not to use Blu-ray, and there's plenty of reasons TO use Blu-ray.
1. They'll need more disc space next gen. That is a given. They can either go with Blu-ray, create an expensive proprietary format, or go with another new format like HVD. New formats generally come with significant costs. It's either Blu-ray or a $599 Xbox.
Of course, they could go with a blue laser variant that's very similar to Blu-ray, and thus would still be relatively cheap, but why do that when...
2. The Xbox is designed to be a multimedia device. Every multimedia device with a disc drive will have a Blu-ray drive by that time. Hell, they all do now. Microsoft will want the new Xbox to stand over and above other multimedia devices, so it'll have to include every major feature found in those other devices.
