There are several problems with Nibiru.
First off, people can't agree where Nibiru is. Certainly the idea that mass extinctions on the Earth are caused by some sort of cosmic shift (something coming through the Oort Cloud and knocking comets into the Inner Solar System) sounds plausible, but it doesn't quite add up correctly. The first idea is that Nibiru orbits our sun, just way, way out there. Well, if the theory is correct, than Nibiru's orbit would have to take him outside of, and then back through the Oort Cloud. This knocks the comets away and is responsible for the extinctions that occur on Earth every now and again.
Second idea is that it orbits another star, the sun's supposedly missing binary companion dubbed Nemesis. Binary or even triplet stars are not uncommon, and in fact it is the single stars that are uncommon in the universe, so the idea of our sun having a companion is quite possible. So, the idea is that this Nemesis star has seven planets that revolve around it, with Nibiru being the seventh. Nemesis goes around Sol, Nibiru goes around Nemesis, and every now and then Nibiru's orbit passes into our Solar System up to Saturn. This shift in gravity forces asteriods out of the main asteroid belt and viola.
There are problems with these ideas.
If Nibiru does orbit our sun and does pass back and forth through the Oort could I would have to wonder just how much astrological power it had. It doesn't even seem to be a part of our solar system all the time, so it shouldn't have power all the time. That idea may sound silly or strange, but that's just how I think of it. Also, this proposed orbital time span is not nearly long enough to account for the cyclical periods of extinction on Earth which are some million in length, not hundred as would be the case of this proposed orbit (closer to a millenia, actually).
The second idea proposes several more problems. First, we have to think about whether extra-solar planets hold astrological significance or not. Personally, I think they do not. Our Solar System is enveloped by the Oort Cloud, and I feel this cometary layer holds in the astrological power of the planets in our solar system and keeps out the interferring power of other stars and planets. If Nibiru orbits Nemesis who orbits Sol, then I wouldn't think Nibiru would be anymore important that Phobos. Phobos orbits Mars which orbits Sol, but Phobos doesn't have any astrological significance, therefore, Nibiru should not. This does not even account for the Oort Cloud, as it is implied both Nibiru and Nemesis are contained within the Oort Cloud and the disruption occurs when Nibiru crosses into the Solar System's Main Asteroid Belt. This supposedly occurs about every 3,600 years, which again is too short to explain the extinctions.
To time these orbits correctly with the extinctions, Nemesis would have to be a couple light years (not AU, lightyears) away from Sol. No such companion stars exist this far away from one another.
Anyway, to answer your question, Nibiru is considered to be concerned with aliens and foreign things like that. Though, that may be thanks to the New Age ideas of Nibiru being a Galactic Battle Station or something like that. *shrug*