Personally I have never had a problem showing my online status in any forum I have ever been a part of.
However like we have been discussing in another thread you do have a right to privacy so there are many options within the Xenforo platform that address these concerns.
I consider following someone the same as a friends option if there were such an option.
You have to understand that you cannot just add something new.This requires addons/mods to the platform.
However as
@Spawn said staff can see everyone online.
It is not "Big Brother" watching you but something that is built into the forum software.