people may get upset if they want to reference an old item in there. if you do go with it, make sure you give notice and tell everyone to save it locally with a lot of notice.
I'd do it for banned members as surely there may be some spam, but not active users. Imagine you log into Twitter one day and your DMs are gone. And yeah, they sent an email they were cleaning house. So what? I'd still be pissed. Leave the messages.