I think either xkcd or SMBC (...Indexed? Bash?) did this better than I could, sometime before, so I'll just paraphrase their Graph/Algebra Joke.
Where N = the staggeringly huge number of doomsday prophecies that have been made to date
Number of doomsday prophecies that have proven true = 0
Number of doomsday prophecies that have proven false = N
Give or take a very statistically insignificant number of "the world's gonna end!" prophecies that intersect, entirely from weight of numbers, with some other random (but non-world shattering) disaster.
My science brain can't help but proclaim "oh and I suppose you think the MMR vaccine causes autism, water molecules have a homeopathic memory, and a 4-metre shift of the tectonic plates under the west pacific causing a magnitude 9 tremor was god's punishment on japan for somehow being 'greedy' too".
As for the 2012 stuff? Trash. We've been through lesser and greater aztec calendar cycles before. They didn't seem to coincide with any major global cataclysm THEN ... why would they NOW?
Answer: Because the makers aren't around to explain the true significance or answer questions, which gives conspiracy wonks license to make up any old speculative crap.
The problem with it is you get a cry-wolf effect, and when someone DOES actually find something that's a genuine threat to the planet, or at least continued human existence & prosperity (global warming, meteors, acid rain, ozone holes, radioactive contamination, whatever) it takes a long time for anyone to actually sit up and take notice, and even then a lot of folk will bundle them in with the "conspiracy wonk" pile. Possibly even until it's too late.