I agree it should be an option for admin for recent to show all previous chats unless you delete the chat I know my users would prefer to be able to open the chat scroll down to somebody they were talking to yesterday and continue a conversation rather than try to remember the persons name and search for them.I see what you're saying but perhaps that should be up the admin. Cleaning out the inactive recent chat list should be included in the cron or even better have a daily/etc timer setting on the control panel. This way it does not affect performance. Just my 2 cents...
I will lodge a ticket with FTP for you