Well I got a chuckle out of the article stating that WN and US would be competing on flights to Provincetown. Either US and WN are trying to capture the gay travel market, or the article's author was busy making wedding plans for himself and his partner this summer on the Cape...
Seriously, though, Southwest does not have to drive US Airways out of PHL to be successful. Remember, their business model works on driving increased traffic by offering substantially lower average fares and far more reasonable walk-up fares. If WN can profitably offer 12 daily PVD-BWI flights, I can see US and WN having enough traffic to fill 12 daily PVD-PHL flights (7 for US, 5 for WN). If US gets its costs restructured and its fare structure fixed, I can see things working out for both airlines at PHL.