well lets not forget that, we dont have to worry about just distance...the number of places to visit in just in our galaxy alone is too big to count really.
so even if you can star trek your way around...or cross an einstein-rosenburg bridge (wormhole) you still have to worry about which of the qazillion places you want to go.
so even if they came here one time...no need to really come back?
id like to think that something from somewhere came by
maybe it was us.
this is a cool site
http://www.ufoartwork.com/