The main reason to use an editor is not included: multi-line commit meesages
I continue to see commit messages by people using IDEs that have long single line commit messages.
A short summary line, which shows as the one line commit message in gitk et al, then a detailed commit messages that can be pages long in some cases is the ideal form.
Some commits only need a 1 line message, but others need long detailed blocks of text. When this is the case, having that short summary, and a blank line separating the lengthy details gives correct presentation in all git tools.