I'm the author. I've made extensions for Chrome before (https://github.com/zertosh/jquery-audit) and I don't think I can incorporate this in a nice way. I'll look into FF. As for a standalone site, I really dig that idea - stay tuned. I'm also learning more about the Uglify internals to submit this as a patch.
I'm the author. Since "a", "b", "c" get recycled by different scopes, it makes it really hard to figure out which "a" you're looking that. By having unique names, you can easily spot where one variable is used exclusively. I was going to use a dictionary but I didn't want the tool to be language specific or run into unsavory words.
I'm the author. Search-and-replace is the main problem this solves. Another neat effect is that if you use something like Sublime Text, you can highlight a word (variable), and see all the occurrences. Makes it much easier to figure out what unminifed code is doing without all the collisions from "h" for example.