While UA is my preferred carrier (I use that term loosely) I fly WN on occasion and their "bags fly free" policy has some benefits in the overhead bin space. That is, even on completely full flights, I have never seen WN gatecheck a bag because it didn't fit in the overhead. I guess they have done the math and concluded that the lost cost of the bag fee is made up by being able to turn the plane faster.

When my only carry-on is my backpack I put under the seat in front of me nearly 100% of the time. Obvious exception for bulkhead rows of course. I do think it's part of the "social contract" to do so. It's really damn irritating rushing to make a connection and your rollboard has to be gate checked because someone's purse. shopping bag, and coat are in the overhead.