I think proper versioning is just as important for themes as it would be for plugins.
Had any site been using a custom theme when we launched our new vertical sidebar, that update would have almost certainly broken their site. It was already a challenge for sites with basic HTML & CSS customisations. We also have to assume that some forums will be using themes that they themselves did not make, but rather a 3rd party did, as is the case for 99% of WordPress sites. Which means they’re at the mercy of a 3rd party should an update unexpectedly break their site.