My under-seat bag holds everything I need for two nights/three business days: laptop, tablet, two clean shirts and a second pair of trousers, socks and underwear, toothbrush and razor (as well as whatever score I'm studying and the aforementioned tuning fork). I don't worry about overhead storage. I get to the airport without having to stand in the check-my-bag line before the check-my-body line. I can take a bump at the gate without luggage issues (which just last month got me enough Deltabucks for a family round-trip somewhere).
Internationally, I generally have to make at least one and (depending on my Euro destination) frequently two connections. Waiting for the checked bag, then customs, then re-checking the checked bag (twice! each way!) is way more hassle than the rare gate-check. If I could fly direct, that would change the calculation, of course.