One of the things I like about git is that I can make bad commits and fix them later. If I'm working on one feature and I'm interrupted by another task, I can commit "wip - blah", then check out a different branch and work on that. When I go back, I pick up exactly where I left off, and amend the half-finished commit into something that actually makes sense before pushing it out to the rest of the team.

In the past, I never made those sorts of commits, because I used VCSs in which you couldn't. Instead, I avoided committing by checking out a separate workspace for the new work. That's a lot slower, though, and it's easier to lose uncommitted changes. Committing incomplete, broken work allows you to leverage your VCS to manage even your unfinished code.

The "Derp" commits are the ones made immediately after a mistake. No one is editing their commit history just to fix these idiotic messages, they're doing it to fix the idiotic mistake that precedes the idiotic message. Yes, no one is perfect, but leaving the previous (wrong) commit alone has real consequences: it prevents people from easily cherry-picking the commit, using bisect to debug later, doing an accurate code review, etc.

edit: I realize that there are probably a few exceptions to my "no one" IRL. But I don't think anyone here would defend that practice.