We (the wife and I) own one of each of the 2011 models.
She uses the 13" because its more portable and its plenty powerful enough to do everything that she needs, which amounts to web surfing, word processing, light spreadsheet tasks, playing music and videos, light photo and video editing, and general family business management. The MBA would have been plenty powerful enough for her too, and more portable, but she needed a big hard drive and wanted to have everything on board.
Even though I actually port the 15" much more than she does the 13", I was willing to sacrifice the portability (and add half a pound of weight over my old Macbook 13") to get more screen real estate and more power to run decently large financial models in a VM and game a bit. To claw back some of the portability I'll upgrade to the rMBP-15 as soon as it gets 802.11ac and reasonable flash storage pricing for 320GB or 512GB, which is all I'll need.
The rMBP-13 makes no sense for either of us (nor anyone else IMHO) as currently configured.