100% of my flying is with Delta so I use their published carry-on size. I do know there are variances. US Airways allows larger carry-ons - 51" total versus the 45" total of Delta.
So the question now is how would they enforce all the variances outside of the gate area? More confusion! "Oh, you are on US Air so your carry-on is okay. But your friend is on Delta and his bag has to be checked". And then there is the airplane variable. If you are flying on an RJ you can probably forget about carrying your otherwise approved roller aboard. I've lost count of the times we've had to gate check our carry-on luggage because we were boarding a tiny Delta Connection RJ.
The whole flying thing is such a hassle these days.
1. You have the TSA and their one-dimensional security theater (I love that description for it). Now they have badges and new uniforms to make them look more authoritative (a Barney Fife answer to a perceived problem).
2. We also see the return to random secondary screenings at the gate with the dreaded BDO (behavior detection officer). That ought to throw a wrench into the overhead bin war games. "No! Please don't pick me! I worked hard to get to the head of the check-in line!"
I really need to get my plan for getting a pilot's license in gear.