Well presumably MS will lose some sales in the Media Center portion of the market.
If a consumer buys a Media Center PC to connect to their HDTV, it is not unreasonable to expect it to play a BD disk as well as a DVD in this day and age, just as you would expect it to receive HDTV as well as SDTV. If MS can't support BD in a consumer-friendly manner out of the box - and rely on klunky, and not that well implemented, 3rd party solutions (Cyberlink and Arcsoft you know who you are) they aren't really taking that portion of the market seriously.
It may be that they have taken a calculated decision that that bit of the market isn't worth the cost. It may be that until the BDA makes it easier to license BD tech (which it recently has) from a single entity, rather than multiple tech rights holders, it hasn't been that easy to do. Licensing from a single entity rather than directly with Sony, Sun etc. may also be a bit more palatable I guess...
There may also be a degree of not wishing to agree that they backed the wrong horse with HD-DVD either.
However if MS think that streaming and download are going to be rivals for BD in the HTPC quality stakes, they are currently mistaken. All the HD downloads I have seen are pitiful in quality (sound and vision) terms in comparison with a good BD.