I just had an occasion where someone reformatted the code in my question, not because it was ill-formatted, but to match his preferred style. I would have liked to be able to type a reason for rolling back the edits when I did so - could the ability to do that be added?
Open up the revision history on the post you wish to roll back.
Click "edit" on the revision you wish to roll back to. Note the message at the top-right of the page:
You're editing an old revision. If you save it, you will effectively roll back to this old version of the post.
Type your reason for rolling back into the "Edit Summary" field.
Click "Save edits". Note that you never click "Rollback".
I agree, clicking rollback should definitely have an rollback-reason or comment input, same as editing does
While you can workaround this by using the "edit" button on an older revision, the rollback button is what I would naturally click in such a situation
I guess a slightly more drastic solution would be to remove the rollback button, and rename "edit" button the revisions page to "revert to this revision". Not only would it inherently prompt for a reason when "rolling back", it better describes what the edit button does (plus gives a nice preview of what the rolled-back question will look like)
Having the possibility to specify immediately the reason of rollback would be a beneficial feature. With no drawback. Leaving a motivation note should be in fact an encouraged behaviour.
The proposed workaround also doesn't work at all if the rollback happened too long in the past, with no error notification (I am allowed to edit and to save, but the change is not there afterwards). For example, I cannot specify anymore the rollback reason for this.
I think the feature should be added. One reason, as already mentioned, is that the approach of clicking Edit on an old revision is not obvious. A related problem not yet seen discussed is that if you click Rollback (not knowing about the Edit trick), notice the summary isn't what you want, research the right way to do it, and finally learn about the Edit trick, you end up in a state where you can't use the Edit trick. Since the post content at this point matches the content you desire it to have, if you click Edit, enter a summary, and click Save Edits, the edit, along with your summary, is discarded.
You can work around the problem by making an artificial change, for example by rolling back to the edit you want to roll back from. Now Edit changes the post content and your summary is saved. It's a cumbersome process, however, and adds noise to the edit history.