I seem to recall in Exchange 2007 if you had a mailbox server that was only used to host a Public Folder database, we'd add a Mailbox database to resolve a specific error. I may be mistaken but I thought it had something to do with Schedule Free Busy information. I'm not sure if this is still the case for 2010. If anyone recalls please let me know, if you have the KB or technet article that would also be very helpful. Of course any enlightenment would be helpful.