Like most people in here, I will get this without hesitation.
Masa I kind of noticed that about the 360 games up to now... I do not own a 360 but I've seen my neighbor finish all his 360 games except Oblivion in no time. At least now I now why... like you he said it did not take away from the quality of the titles.
On a personal not I just hopwe it does not become a trend. I like my games to be at least 20 hours long, 30 hours + would be perfection.