I imagine this is just marketing on Apple's behalf. This revision has brought the 17" closer in price to the 15" as it's ever been, and putting an i5 processor in the stock 17" makes the price gap seem even smaller. All things told, my 17" i7 AG was exactly $200 more than a 15" high-res AG which felt like a no-brainer for me.
As for the portability factor, the 17" actually fits my old 15" laptop bag just fine. It might be huge compared to a 13", but it's really not much bigger than the 15". And while I initially also thought it looked a bit funny with that "small keyboard floating in a sea of brushed aluminum" effect, that went away once I took it home and started using it. It just looks very proper to me now