The convenience of traveling on carry on only (and I can do a couple of weeks of work and/or leisure in a 19", handle and wheels included, rolling bag that weighs in loaded at 10 kg/22 lb, and I am a pretty high maintenance female) is worth a fee. It's not the fee that keeps me from checking luggage, but the inconvenience.

Assuming I manage to maintain some sort of status with Delta I'd likely not have to pay a carry-on fee in any case, but it would still be worth it for me if I did.

And if I did have to pay a fee to use the overhead, and if that fee were prohibitively high I'd just find a way to pare my luggage down to the point that it would fit under the seat.