The general theory that goes hand in hand with chemtrails is that 'the elite' are utilizing them to make us ill, keep us exhausted, and dumb us down so that we are more easily manipulated and they can further implement their agenda of global domination... but this makes little sense as we're all breathing the same air.
I've heard chemtrail believers often say, "Man, they sure are spraying us today!" However, I think that has more to do with altitudes and pressure systems than anything. The overall amount of air traffic in any given area is generally the same, but if it's taking place on a day where there is a lower pressure system in the area and clouds are more likely to form, then you're also more likely to see the contrails of aircraft spread wider and faster. Would I want to get up behind a 747 and breath in the exhaust? Fuck no, but the idea that some days are 'heavier spray days' than others seems laughable as well.
That said, I firmly believe that having as much air traffic as we do globally is causing a multitude of problems with our air and atmosphere, but the idea that it's an evil plot is again laughable. I think it's an inevitable consequence of the global industrialized age we live in.
I'd like to end with a list of patents I have found being tossed around the net like a cheap Vegas whore. Make of them what you will. I haven't done enough homework to really say for certain what any of this means. I have noticed that cloud seeding, weather modification, crop dusting technology (because God knows our corn is much healthier when it's coated in chemicals), and various patents to prevent 'global warming' pop up quite a bit and have so for years.
http://www.geoengineeringwatch.org/an-e ... f-patents/