My only issue with the "hitch a ride" view of the aliens is that it doesn't make them very threatening. I mean how much of a danger can they really be if all we have to do is go "oh shoot, aliens on this planet, don't go there".... and that's it. End of story. Drop some bombs on them. The great menace of the cosmos... not much of a menace after all. Now I'm not saying that they could actually build ships. And probably didn't build the derelict (seriously, I tried to reason that one out and people found all kind of hole in it, so it ultimately doesn't make a lot of sense). But I don't leave out the possibility that the aliens might have someone of spreading through space. Maybe after consuming a world then enter into a next phase of development (the best I have been able to come up with is they find a way to spread some kind of proto-alien spore into space, which becomes and egg once it lands on a planet that supports life). Or maybe an strain of aliens has been growing and evolving since the time of the space jockeys and they have now developed a form of space travel. So the aliens that have been encountered in A1-AR are the equivalent of our cavemen to those more evolved aliens. But crazy ideas aside, I'm just not fond of the idea that the aliens are just nastier-than-average ship rats. Because then the aliens just have to get really lucky and hope that someone stops by to say hi, which, considering the vastness of space, becomes extremely unlikely.