I think "support", for video codecs, would mean shipping with the browser. Adoption will be very limited as long as it requires a separate download and installation (unless the browser makes that seamless, with no admin password requirement).
How is adoption going to be limited if Chrome and Mozilla both ship it? That's 50% of the market that get it w/o thinking. From all indications MS wants the codec to work well on their platform. They don't want the liability of shipping it.