We have a very common customer scenario where the customer decides to upgrade to a new version (in this case let's say SQL Server 2012). They run upgrade advisor and note that there are a whole lot of problems. They think "Hey that's way too much to deal with right now. I don't have time".
So they don't deal with them. They upgrade the server to 2012 but leave their db_compat level at a lower level with the intention of dealing with the problems later.
But when they go to run the upgrade advisor later, to find the issues that they need to resolve before changing db_compat level to 2012, it tells them that they can no longer use it because their server is already at 2012. How can they now check what needs to be changed? They also cannot now reattach their databases to an earlier version server, as they've been upgraded to 2012 structure.
I believe that upgrade advisor should check db_compat levels, not server versions, before refusing to run. If you agree, you know what to do. Vote once, vote often: