Trip pay is a "legacy" thing -- that's the way it's been done before, and it continues to work. Could SWA convert to hourly pay? Of course they could, but at what cost? Right now, everything contractual (both pilots' and flight attendants') is done based on trips. Doing a conversion when both contracts aren't being negotiated simultaneously would be a bear. Things that are now easy round numbers (6 trips per reserve day, 6.5 trips average daily guarantee, 5 trips duty period minimum, etc) would become ugly numbers (4:32 duty period minimum, etc).
Also, there are some unique features to the trip system, one of which was touched on, that the first 12 minutes are "free," which reduces the incentive to taxi slow for more pay, without removing the extra pay when a flight gets well & truly delayed. There are some other interesting twists to the "trip" system, such as no flight being worth less than 1.0 tfp (trips for pay -- the official term), and a balance between the "time" and "milage" formulas so that crews get the better of the two (i.e. pay based on distance going LAX to BWI, and on time going the other way).
Then there's the $ cost to reprogramming all the scheduling and payroll computers to think in terms of hours for pay rather than trips. Computer programming for large, complex systems isn't cheap!
Could all of the above be dealt with, adapted, and overcome to change the "trip" system? Of course. It's just not on anybody's radar screen to do it right now. The costs (both monetary, and in terms of confusion) seem a lot higher than the benefit of "having an easy comparison" to everybody else. The standard conversion formulas work well enough for most purposes.
Snoopy