Apple Mail on OS X has a peculiar way of working with the drafts folder, and so does Gmail, and the two don’t mix well. Apple Mail automatically saves a copy of a message that you’re working on every 30 seconds in the Drafts folder, and then tells the server to remove the previous version from the Drafts folder.
The problem is when Gmail receives the instruction to remove the previous version of a message in its Drafts folder, it automatically moves that previous message to its Trash. The result is that your Trash immediately fills with many, many iterations of the same message.
If you designate a folder other than the [Gmail]/Drafts folder for Drafts in Apple Mail, this doesn’t happen, but the results are arguably worse, which is that when the partial draft message is removed from the folder by Apple Mail, it hangs out in the Gmail All Mail folder forevermore (which you might normally never see, but this could be a nuisance if you need to go searching for it).
So I simply don’t save my Drafts messages folder to server when the server is Gmail or Google Apps. I disable that setting in the account preferences in Mail. I don’t get my drafts synchronized to other devices, but I can live with that. I leave the [Gmail]/Drafts folder visible so if I have any half-composed messages I wrote on the Gmail web, I can still see them in Apple Mail.
But if you really want synchronized Drafts, and you want the folder to behave correctly, you can go to your Gmail settings and under “Forwarding and POP/IMAP”, turn Auto-Expunge Off, and in the following section choose “Immediately Delete The Message Forever”. If you choose this setting, you must turn off Archive Messages on any iOS devices (see my previous post on this) or you will lose a message forever when you tap the Archive icon.