I can see why microsoft is doing this.. It has a myriad of reasons behind it.. But lets get to what I believe is the primary reason: They want all of their platforms to use the same market place.
Microsoft is pushing hard for unity across their platforms, phone, pc and game console. While this works out imo on the phone/pc front, I dont think the general availability of high speed internet (by that, using the 2012 definition of it, which is IMO 12+mbit down stream at a minimum) is there yet to support a purely cloud based purchase of video games. Especially considering the sheer size of some games today, with no chance of getting any smaller. It would seem impractical at this point to switch away from a storage medium for game purchase.
That said, I've seen Microsoft make some bonehead moves before, but never something that is flat out stupidity. They have a console lead at this point, and it would be purely bad business to try and drastically reinvent the console at this point in time. Let Sony try that and get people primed, then move to it during the life span of the 720, then at the next refresh, go this route in about 5 years.
I think the rumor of the storage-less (in terms of game acquisition) are not true. I also believe it will have either an internal optical drive that is compatible with the 360 games, or an add on drive that can be used if the elect to go with a SSD game delivery medium.