If you click on that, you see also the guests and if there are a lot of guests coming and going and doing some other stuff, the list gets completly disordered... I tried it here where are 200 users and guests online... The same member was on page 1-3 just because a big guest activity.. so the link/list wasn't useful.
I suggest a checkbox or a separate site to enable / disable the listing of guests.
Or a sorting, where guests are at the end or something...
Edit: Argh is already suggested sorry. For any interests: http://xenforo.com/community/threads/suggestion-who-is-online-page-display-order.954