I really live having good formatting and spell check in the commit messages. It's much nicer than the `git commit -m "blah"` messages. Sure I could use another GUI, but magit is very easy when I'm already in emacs.
If you leave out -m 'blah' git uses your $EDITOR which could be vim/emacs/whatever, which of course can spell check. So that one part is not a selling point for magit.
Perhaps you do not know, but if you leave off the -m switch, git will launch $EDITOR with a commented list of files changed, where you can write your commit message.
Perhaps he did not know, but I find that a lot of developers, especially those not using GUIs, end up using -m every single time and writing poor commit messages. With Magit, at least in my experience, you are more inclined to write better commits.
If you give developers a "lazy" option, a lot will default to it, which isn't ideal. -m is an alternative for a quick commit and end up being front and center when using the CLI.