I guess it all depends on what degree of prepping they do. Natural disasters happen with regularity, and being ready for unexpected problems and protracted loss of infrastructure (Katrina, Sandy) seems wise to me.
I don't think we have anything to worry about. The preppers seem mostly harmless. Probably a good idea to keep an eye on them though.
There's still the impending zombie apocalypse!! =D
Any idiot who thinks building a tiny bunker with a few years of canned food and a lot of guns = salvation from the global apocalypse, should go in their bunker and wait while we weld it shut.