While indeed unchecking this option stops the behavior, it's not the best of solutions. The options to save drafts on server is available with any decent IMAP program, and generally is very beneficial. Say I start a message on my Mac, and then have to leave before it's done. If it's saved to the server, I can then pick this message up later on my iPhone or iPad and complete it and send it. Or I can start it on an iOS device, have it save to the server from there, and then pick it up on my Mac later to complete.
So, the functionality of "Save drafts on server" is widely known amongst IMAP clients, and it would be nice if Apple Mail would simply replace a previous draft of a particular message with the latest version rather than piling up multiple versions of the email. Also, it's customary that when you eventually send the message, the drafts also disappear.
It would also help if there was a way to increase the auto-save time to a longer setting so that they don't pile up so fast.
Danita