Boeing Boy:
You make excellent comments that often get glossed over. So here is the deal.
Connecting traffic is inherrently less profitable than nonstop traffic. The $99 transcon trip with a connection anywhere is more costly to produce than the $99 transcon nonstop. However, the amount of traffic from most places on the east coast cannot justify a nonstop flight by any one airline. Thus, the hub and spoke system was born.
The low amount of O&D traffic, as you noted, puts pressure on yield. This is because, since connecting passengers are by their very nature lower revenue (because nonstop service is considered a "premium" product to connecting service, and more competition as you noted), and higher cost. Therefore the local traffic (i.e. PIT to LAX) gets a premium service (multiple nonstops to choose from), and should be charged accordingly (higher fares).
The problem with PIT is that with such a small total O&D traffic base, the amount of "high-fare" local traffic is extremely limited. So you have two choices... Get more people to pay high fares or raise the fares. Since PIT as a city has been shrinking in the past few decades, getting more pax to pay high fares for the premium service has not happend. Thus the fares at PIT have risen.
The next problem is that when the fares get too high, folks begin looking for low-fare options. Folks on the fringes of the area served by PIT start to find other airports to use (i.e. CAK, CMH). Low-fare carriers enter the market (i.e. Vanguard, AirTran, America West, ATA) because they can undercut, offering an inferior product (i.e. connecting service) at a lower cost to the consumer. This has a two-fold effect, it lowers the yield the hub carrier can choose and it reduces the O&D market size. When this happens, the hub carrier can either lose money competing or raise fares.
So, as flights are added to a Low O&D hub, the amount of connecting traffic as a percentage of total traffic increases. This measn that the revenue per passenger on average declines, because the Low O&D hub local cannot support a fifth frequency to LAX when it can barely support 4 (for example).
Now you take a situaiton like PIT, where the average revenue per pax (both local and connect) is declining because there are more connecting passengers, and cost per passenger is increasing, and you end up with a loss-making hub.
Consider this. Back in the early '80's, some of aviations forward thinkers began to consider the economics of a hub in a corn-field in Iowa. The idea was to build special airports in the middle of no-where which were designed as transfer only points. Costs would be reduced because there would be no ticketing, no baggage claim. Only Connecting Passengers. Only runways and concourses. The problem with this was that it created a hub with all low-fare, high cost connecting traffic. So while it would make sense from a lower costs by not building/staffing the landside areas, it did not make sense from an operating standpoint (cost per passenger for the operation).
Now back to PIT... The US Air PIT hub, from what I have heard, is around 80% connecting. That is extremely high. I have heard that about 40% of Southwest's system traffic is connecting. Combine that with the fact that the cost per passenger at PIT is very high compared to other airports, and you see US Airways’ PIT problem.
You are right... There is an impending problem at PHL. I believe the PIT hub is currently larger than what the local passenger premium pays for. In other words, PIT's losses are because the hub is too large. CLT works because it is a low-cost airport. If you are going to have a hub with 70-80% of your passengers connecting, you need it to be as low-cost as possible. This is why CLT works and PIT does not. However, if the CLT hub becomes larger than the O&D traffic can support, the same thing will happen there.
So when I refer PIT not having enough O&D traffic, this is what I am referring to.