[zippergz]

That certainly contributes. But I can check my bag for free (as a frequent flyer) and I still never do for several reasons. It saves me a lot of time, being able to walk directly off the plane and to my ride rather than waiting half an hour at baggage claim. It is impossible for my bag to get lost or mis-routed, which is a major hassle when I'm on a short turnaround business trip. And finally, it makes it much easier for me to switch flights at the last minute, which I do with some frequency when there's a delay on my original flight. These things are small, but they add up when you fly a lot.

My bag is not oversized by any means, by the way, but it does need to go in the bin, not under the seat. That's what they have bins for.