In my view a changelog is so fundamental there shouldn't even be a debate on whether or not one is needed. We're talking software behaviour here and I, for one, want a heads-up on what impact an update may have before it happens.

I check plugin changelogs beforehand and this has saved me from several embarrassing moments and at least one critical event. Not so with themes unfortunately :o(

I remain frustrated that this basic document management tool is not present for themes. I have resorted to running fully-functioning and up to date localhost sites in parallel with those online, so I can update themes in a managed environment first, and get to the changelog before updating for real.