Parmandur
Book-Friend, he/him
Because when it came out, that's all people talked about. It's pretty obvious that the subclass was designed to let players play the archetype of one of the most popular anime series we've seen in decades, and mechanically, it clearly was trying to capture that. Thus, the many comparisons to AtlA that are all over the internet in regards to the class.
No, that would be redundant, and we certainly don't need a ton of subclasses that are pretty much all the same, except "swap fire for water". Also, having all four elements into the same subclass allows the PC to actually emulate the Avatar by having access to all four elements. Which they couldn't do if each was a separate subclass.
In Avatar, Bender's are only tied to one Element, with the exception of the Avatar. To get the feel of Bender's, properly, one for each is appropriate. The Avatar proper is more of a Sorcerer, with a couple Monk Levels.