Empty a metadata field at revision checkin

An issue that dogged me for ages was that metadata fields can't be reset to empty for a revision checkin if they already have something in them. For example, if a content item at revision one has some comments then the checkin form for revision two will display the same comments. There's no way to clear the field for the user. Even if they delete the comments and then submit, the comments reappear! The only way to get rid of them is to type something else. But there is a better way.