I will have to disagree on this one. What are the reasons to go back? The big one is that problems pop up in the new version and you need to go back. But, isn't that what testing is about? Test the database on the new version of SQL and find those issues before migrating the production database.

Are there are other reasons to go back to a prior version?

What kind of support nightmare do you have when upgrading some customers then moving them back?