I guess sony is looking for a long term goal with ps3. They expect the ps3 to last as long as 10 years, and including blu ray will help the life span.
Beside lenghty FMV, you can include Dolby TrueHD surround sound audio tracks on the High Def video, even though vc-1 and mpeg-4 are much better compressor, but using higher bitrate for 1080p fmv should yield better picture. And the extra space on blu ray, imagine playing videogames that have behind the scenes on the blu ray, trailers or ads, and interview with the developers all on one disc and other features. So the extra space won\'t hurt either.