If I'm traveling alone, on business for just two or three nights, I'll usually just stick to a carry-on size bag (with wheels) and a laptop case/briefcase. It's just faster.
If I'm traveling with my family, however, we always check our suitcases and just carry on what we'll need on the flight or don't want to pack: backpacks with snacks, books, etc. for the kids, DWs purse, camera case, etc. There's just too much stuff to pack to fit into the smaller cases and hauling around four carry-ons, plus backpacks, purses, etc. would be a HUGE pain.
In my experience (which includes being on 100+ planes over the past 18 months), leisure travelers are much less likely to do the carry-on thing. I assume for the same reasons I cited. If I'm on a weekday flight to a non-tourist destination, overhead luggage space often runs out. When I'm travling with my family to Orlando or another tourist-type location, the overhead bins rarely are full. That's not to say that it doesn't happen and I don't fly SWA enough to have a read on if they're different, but that's my observation.