The 15" is one of those machines that's so beautiful if you know how to use it, that the extra weight wouldn't matter at all to the right user.
If you use the functionality of the 15" like the dGPU, quad core processor, extra screen real estate, then portability is nothing. It's totally worth the extra weight.
But if you never benefit from the dGPU, quad core processor, and aren't watching a lot of movies or running multiple programs on that gorgeous 15" display, the 13 inches' functionality is enough and the extra portability is what you should be looking to take advantage of.
I have a 15" Windows laptop that I have to bring on some jobs, and that thing is heavy. The 15" rMBP is noticeably lighter, and I carry it with me the other 90% of the time when I don't need the Windows machine. That Windows machine, despite being labeled as a "gaming" system is a dog in performance compared to the rMBP as well. So for me, the choice is between a fast, lightweight machine, or a slower, bigger, heavier machine that I don't even enjoy using.