Thinking about it, derailleurs are for shifting, right? In the rear you have more gears than at the front, and most likely will be changing them more often. Thus, having a better RD than the front one sounds logical, no? I'm asking because I see a lot of bikes with RD being somewhat higher end than the FD.
On the same note, how often do you shift in the front? For example, every time you're on a flat section, do you upshift to the biggest gear, or keep it in the middle (on triples) and just upshift at the rear?