It's more streamlined, it's easier to scroll through staged and unstaged changes fast and flip lines/hunks/whatever between the two as you go, and you don't have to fire an external editor upon each hunk because you're already in one - emacs or vim (if using spacemacs).
(Also, whether it's good or not, rebasing in all of its forms becomes your second nature because it becomes so easy...)
If you're selecting by hunk, you don't need to fire up the editor. I also find the hunk selection in the terminal to be excellent. It's lightning fast and I can use it blind.
Also, firing up the editor for things like editing diffs when doing git add -p or when editing operations when doing git rebase -i, is not that big of a deal if you keep your editor light. Vim, for me, loads in an instant.
Agreed, selecting by hunk is ok in the terminal, I do it too sometimes when sshed someplace distant.
But you noted the two main differences yourself: (1) you don’t have to think in terms of git hunks if you don’t want to, lines or any blocks are as easy to deal with and (2) you don’t have to think about whether it would be quick or not to fire an editor because you don’t have to fire an editor.
This, plus everything’s lightning fast due to shortcuts. I use Magit in spacemacs and rebasing is very easy. I’ve done it so many hundreds of times that I can tell it from memory - e.g., fixup-merging the last two commits to the third one: “lljjriff,,” (maybe there’s a quicker way) - that’s less than typing git rebase -i.
Not trying to preach, just sharing why magit users are so happy about it.
(Also, whether it's good or not, rebasing in all of its forms becomes your second nature because it becomes so easy...)