I think at the moment it is a hypothesis, not a theory. A theory has results that can be proven by more than one experimenter. A hypothesis is an idea of how something works, but has not yet been tested or cannot be tested yet.
That being cleared up.... lol
I think it's very plausible for Planet X (or Nibiru or whatever you want to call it) to exist and be in our solar system. I mean, it's not like our scientists are infallible and know every single thing about our small part of space, even though many like to think so. Not to mention, our gov does cover up tons of unnecessary things from the public, so that part of the hypothesis has been tested and proven and we can't rule it out in the instance of the planet x hypothesis.
The other tricky part is the part of the hypothesis that suggests Planet X cannot be seen until it's nearly on top of us. If this is true, it's very likely that in the event there was not a cover up, no one would know it was coming until it passed Jupiter or Saturn anyways and then it would be only a short period before it would begin to pass us.
As to the weather anomalies and natural disasters caused by it come near us, it would probably have to be closer to us then the moon, in which case it would probably hit the moon and that would cause more than enough damage on it's own.
Personally, I don't think it will come as close as that, if such a planet exists and follows the planet x hypothesis.