Up until late 2004, PIT was a full-fledged hub for US. It's still possible on many routes to connect in PIT, though flights from there to many destinations run with far less frequency than those from CLT and PHL. I still connect in PIT whenever possible (though from RIC it means at least one segment, usually two, on an RJ). The reasonably priced restaurants, the wi-fi, and the lightly loaded flights are why I prefer PIT to CLT. I've transited PIT twice in the past month and both times the airport's been deserted, especially around the US Airways gates. Southwest's flights to MCO and MDW appeared to be completely full, though, based on the crowds of people I witnessed lining up to board at their gates on A concourse. Seems like much of Pittsburgh's O&D traffic is now using airlines other than US...even the DL and NW gates on D have more pax waiting at certain hours than some of the US gates.
PIT-RIC flights must be a huge money loser for Trans States/US as all flights I've taken on that route since the dehubbing of PIT in '04 seem to be at least half non-rev (pilots and FAs commuting between Trans States' RIC and PIT crew bases, plus many pilots from other airlines who live in Richmond and connect in PIT to wherever their base may be).