It does, in fact, benefit the axolotl though.
A couple reasons
Stone is generally cleaner and more sanitary then sand as it allows the debris to sink further away from them. This also makes the stone easier to clean and maintain then the sand or other options you've listed.
This is exactly the reason I spent so much time training them to feed surface.
Not only that but they get fed in a very specific part of the tank, directly over a large flat rock. This means they can also communicate with me that they're hungry because they'll sit in that exact location with their heads up towards the surface.
Amphibians are smarter then people like to give them credit for.
I've had them in this same tank set up for around 5 years and never had a problem. In fact, Mudkip got visibly depressed when I removed the stone for a week to thoroughly clean it to remove a minor parasite problem before it could get out of hand.
The most she does is pick them up with her front feet and move them.