When should I use shared mail?

Never. I'm referring to the single object message store here.

If there is anything I learned in my tenure at Lotus, it's that friends don't let friends use shared mail. Sure, shared mail will give you an opportunity to save some disk space. But that comes at a very high cost. It's a complete pain to monitor and troubleshoot. Not only that, when the occasional user deletes and item from their mail file, or if they delete their complete mail file, the only way to recover it is to recover the entire server.