Comment 43 for bug 861171

I agree that there isn't much point in limiting who should be able to shut down the machine when accounts are logged in, when the user has access to the power button anyway. I feel that any user should always have the opportunity to "cleanly" shut down the machine via the GUI, not allowing that is a big usability issue. Ubuntu should first warn that other users are logged in, and the offer to shut down anyway, regardless of their account status.

For a more long-term goal, it would be nice if ubuntu offered to log the accounts out for you, and in doing so saved the sessions of the logged in accounts, so no work is lost when the machine is shut down, but it would probably require a lot of work to make that a possibility. In the mean time, simply giving a clear warning that work will be lost if the machine is shut down while users are logged in, and then letting the user click shutdown is sufficient.