Well, until Apple offers resolution independence some people will find the user interface a bit too small.
So 1680x1050 on the 17" screen would be similar PPI to 1440x900 on the 15" - bit more comfortable on the eyes.
Don't get me wrong, higher resolution is great, but some people like me, find things just a bit too small, and it would be nice if Apple provided a way to change the DPI.
Quite so.