I suppose the ground travel gets annoying if you're bringing along someone (like a wife) who isn't quite so into the MR game. I've started solo MRs at a few other Midwestern cities after taking Amtrak or a bus (Greyhound or Megabus) to get there. SEPTA's infrequent, overpriced R1 does make PHL one of the worse-connected Northeastern airports, although it's pretty quick once on board.

The DC-Philly buses disembark at or near the bus terminal, which is next to SEPTA's Market East station. Not quite so seamless a transfer as Amtrak to SEPTA at 30th, but not bad. Of course, you can't really plan a schedule around Chinatown buses in particular.