How do you know a truly awesome coral? it's almost impossible to keep it alive.
How do you know an obnoxious weed of a coral? It's easy to keep.
So says a reefer with a guilty love of pulsing xenia (which ironically disappeared from my main display tank at the same time my acropora started to do better). Maybe acropora are the problem.
Just kidding of course. It's often best to try to keep potentially fast spreading coral isolated and under control because they do have at least the potential to take over a tank. This is particularly true of corals that are not necessarily easy to trim and remove.
As far as aiptasia go, if I could turn back the hands of time, I would have been much more careful about trying to keep them out of my main tank. I have a filefish that keeps them in check and doesn't seem to both my coral (although I don't have any rock flowers in my main tank anymore

) but I always feel like I'm on the verge of a potential plague.
Matt