I think it's a very good idea to be prepared for disasters (though I'm not so much). Bad things happen and the people who can take care of themselves free up official resources to assist the people who most need it.
That said, I don't like the mentality of some preppers. Some seem to be so anxious for the end times to come that they wouldn't mind helping it along a bit, and they have the weaponry to do so. You would think the preppers would be the ones helping out others in a disaster but I get the feeling these would be making matters even worse.
A lot of the prepper scene is dominated by marketing. I always laugh when I see their "bug-out bags" loaded down with so much junk they could hardly carry the thing.
There is a spectrum of what you could consider "prepping": From having some canned food in the cellar to an underground bunker filled with enough weapons and ammunition to start a revolution. Of course the ones we hear about most are the nuts. I know one of the things the HAM radio people are into is providing emergency communications during a disaster.