The 13" Macbook is certainly the better bang for the buck, but the smaller and lower resolution screen is an absolute deal breaker for some, including me. I'd imagine that the better screen on the MBP 15" is the #2 reason folks would choose the MBP, right behind "work is paying for it."

Then again, there are probably many folks who will choose the 13" even if they're not footing the bill because it's more portable. It's all about how you setup the utility function, I guess.