I've been working with iOS 7 and I recently realized that there was a folder called documents and data on my iphone which was 2.1 gb. Apparently these are my imessages. I decided to delete all of my imessages but every time I attempt to delete about 4-5, the app stops registering that I've deleted them and if I reopen imessages after closing it crashes and displays no messages and opens up a new imessage. The only way to correct this I've found has been to restart my phone. Clearly there is still an issue here. Let me know if this happens to you as well