Yes, it is useful to see if a user is "active" or not (not based only on his activity ). Some users come to the site but they don't post on forums, comments... they just log in and browse and logout.
Last Seen / Last Log in
Make Regular users able to ( and using the option of "'user_allow_viewonline' ) see or not if profle owner is online, or something like: Last Log in: 3 hours ago. If The profile owner is hidding his status then show just "offline".
For Admins Show Regular info like: Online, Last log in: Yerterday. Or Just Date and Hour.