Why not? Where else do you want it? Is something forcing you to read the full commit log?
There's no length limit on commit messages and commit messages are mostly out of the way. Most VCSes have a way to only show you the first line. So if you want summaries, that's what the first line is for. If you want the full story, that's what the body is for.
Combined with annotate/blame, commit messages can be very helpful source-level documentation. Nobody has ever complained about too much documentation, and commit messages are the perfect time to document what happened because it's one of the few times where our tools actually force us to write something in order to proceed. As long as we're being forced to write something, write something good and informative.