There's a good chance the person you are replying uses git commit --amend all the time to modify commits in conjunction with git add -p.

No, hg amend -i is much worse than git add -p. I do that when I have to use mercurial and there's significantly more mental overhead to make sure I'm not accidentally modifying a commit I did not intend to modify. Creating a "temporary" commit before I have a complete change is also annoying.

And once you need to add a file, you're back to having a staging-like concept.

IMO the staging area is fundamental complexity.