I generally do that sort of thing(staging/destaging lines and chunks) in my editor. If I could shift the merge tool stuff into my editor quickly I would do that as well. The reason is that it reduces context switching and difference between environments: sure, I could spend hours making the fonts, colours, and keybindings the same between the git gui and my editor; I could get really good at switching to the window; etc. But even a slightly worse version of it in the editor is going to be way less work.

At least, that's how I see it.

Also, I'm sure there must be a PPA or something for gitg on Ubuntu, if you really like the tool.