"Need to be stickied" is a bit of a misnomer, to be honest. Any member who can afford it is welcome to sticky a thread. It's probably most applicable in the WOW forums, where people want their thread to be continuously viewed. Threads that are constantly active don't need to be stuck anyway, as they're active. When a sticky thread becomes inactive for a period of time, it'll become unstuck and will drift down the page like any other inactive thread.
In short, there's not an awful lot of point in buying it, to be honest. Unless you really think it's worth it, or you've got your mind set on it.