:: I wonder if there are other low-status jobs that only waterbenders or earthbenders can do, or if that's unique to firebenders, since the Fire Nation was the most industrialized of all the four nations. ::

Remember the earthbenders who moved the trains in Ba Sing Se? And there was similar stuff going on in the Northern Water Tribe. I've long had the impression that benders are more like engineers/mechanics/technologists/construction workers in this universe. People who build and run the infrastructure. Even what the benders did in wartime had a lot to do with that engineering/mechanic role: the big advantage in having benders around was in what they permitted in the way of war engines.

Against that background, the Mechanist was a little odd (a nonbender inventor-technologist!) and it makes the Satomobiles very interesting. (They don't need benders to make them go! Anyone can use them! I'm dying to know who Sato was.)

In fact, I kinda wonder whether that kind of bender-free tech is one of the things that make the Equalist movement possible: it's theoretically possible now for society to get by without benders.

(Or so they think. It's usually harder than that to just wholesale remove a sector of technology like that.)