I own a 17" MacBook Pro. It's definitely big, but surprisingly easy to carry around, and I'm not a big guy at all. I love 1920x1200 (DO NOT SAY 1080) and the big screen makes it easy to throw apps all over the desktop and have them still be viewable. It's not heavy at all, what's it like 6 pounds or something? My Dell 17" is much heavier.
My suspicion is Apple wants to go Retina in the MBPs, and they want to use this opportunity to switch the panels to the same sizes as everyone else. The 17.3" 1920x1080 panels are the "in thing" in the big laptop PC world, and Apple must be taking a beating on the pricing for 1920x1200 panels since they are the last laptop maker using them. Going to a 17.3" panel may be unworkable due to the thinness of the MBP or some other design flaw that won't work at that size. So, switch everyone over to 15(.6)", forget the 17" ever happened.