As someone who has spent a lot of time tracking down the origin of source code changes -- and the reasons for them, and the implications of the change -- by reviewing commit logs, I can think of little worse, short of no commit history at all, than trying to derive anything remotely useful from a commit history that has been condensed down to a product change log.

Even commits that lack good commit messages provide valuable information in the form of insight into the author's cumulative thinking/process.