I think it could be argued that SYR (or ROC or ALB or MHT or PVD) is also perfect RJ territory; the company faces some very difficult choices. One one hand, cities like ABE, AVP, MDT, DAY, etc. struggle to draw the number of passengers US Airways needs to be profitable, given the fares charged and the company's costs. The RJ's increase average fare level with lower trip costs, but also have much higher ASM costs. The big problem with this strategy is that the company ends up being less competitive long term in the markets that are shifted from mainline to RJ service. As others have said, DL, NW, and CO then offer equivalent or better products/schedules, which makes it difficult for US to retain passengers. And if one of the discounters like WN or FL enters the market, the RJ strategy really gets blown out of the water, since you simply cannot compete on costs by flying RJ's to the hubs.
I feel that one of the dangers in the RJ strategy pursued by the majors these days is that they are using them to replace former mainline service, rather than to improve feed to their hubs. I believe that improved hub feed was one of the cornerstones of DL's success with RJ's at CVG and CO's success at IAH and EWR. And to a large degree, US Airways was a major contributor to DL's and CO's RJ success story, since both were able to steal passengers away from relatively unattractive turboprop service, with US Airways having been hobbled by restrictive scope clauses. While RJ service should replace mainline flights in certain cases where loads are poor, even with reasonable fare levels, relying on lots of high-cost RJ feed to the hubs makes it more difficult to compete with LCC's who may enter the market.
I have to believe that WN entering ABE (as has been rumored in the past) would be troublesome for US. They have the potential to draw travelers from the Philly suburbs looking for a break from walk-up fares on US. Then again, it's possible that ATA and AirTran are already doing a significant amount of damage at PHL as it is.
