I posted something like this in the MBA crossover thread, but since this thread is rMBP crossovers and that's actually my facts, here goes.
I walk about two miles and have a half hour train ride to work every day, with the same coming back home.
Right now I'm carrying a 13" rMBP and an iPad (was an Air; it got too big and heavy so I changed to an rMini) every day. It's ok, but I notice the weight. And at work I don't need (actually can't use) the USB ports for data exchange. Last week I was experimenting with an 11" MBA instead of the 13" rMBP, which is a weight difference of a little over a pound, and it was very noticeable. I also don't think that the rMB would be challenged by what I do with it while commuting or at work - mostly Word, Excel, Outlook and Safari.
So, getting rid of almost 1 1/2 pounds of laptop (vs. the 13" rMBP) with no give-up in screen quality or connectivity (in my case) is really no issue at all. The slightly smaller screen might or might not be an issue under some conditions, since I often need to have two documents open at once and the 13.3" screen might be just enough bigger to make a difference - hard to tell without trying it in person.
The biggest issue is whether I feel like putting $1300 into that dedicated use (and keep the rMBP), or justify the $1300 by selling the rMBP (not inclined to do so - and I do things with the rMBP at home, like photo and video editing, that I'm not sure the rMB is a good candidate for). A secondary issue is whether it's dumb to buy a Gen1 design. The original Air and iPad were innovative but not fully developed (duh). Resale value is probably high enough that I might take that risk.
