I have been developing for over a year on a MBA 13" with no problems using Xcode, Photoshop, QTPro 7 and FCPX. I also use a 13" 2010 MBP for some animation work in Windows (Bootcamp) and FCPX work.
Now with the RMBP I just have more screen real estate and everything compiles and renders much faster. One of the greatest things with the new MBP is the USB 3. Transferring files and backing up to external drives is so fast.
Didn't need it but love it.