This seems quite handy from a staff perspective.
It's a fairly standard option on many systems. However from a personal perspective I can't see it being that useful, not on giffgaff anyway.
The thing is if you are online what are you going to be doing? posting publicly and a quick glance to the left on your about me page and you have a timestamped history of when you last posted so even if you are appearing offline it doesn't take a genius to work out that you're not really offline.
I agree Hazel that you'd find it useful because you read the boards (and I spend a lot of time reading I must confess) but for most members if they are online then they'll be interacting so the hidden online status won't really achieve much but for an educator it would be useful.
If the idea was aimed at staff (which I don't believe it originally was despite what the OP claims) then I won't kudo it. I like to know when there are educators etc online.