

Well, you might be inclined to not roll the feature out at all, depending on the results you see from the rollout/an A/B-test. Also, having it written out with a date in the changelog binds you to that date, unless you want the embarrassment of not shipping on a promised time. Maintaining a changelog for very large app development organizations is also a pretty damn hard task, trying to coordinate whatever all teams are releasing in a particular build.
I agree that getting cute with the changelog messages is a bit stale. Might as well not add anything at that point.
Pick-Up Group