Touchy issue...
  Personally, neither. I argue, because 
 1) I view it as a game. It stimulates my intellect, and sense of logic, or so I like to tell myself. It also has the nice benefit of brushing up some of my less-present knowledge (the Evolution debate, in various forms, recapped my learning of dating techniques, for example). 
 2) I learn new things. For example, Skin explained some things about aforementioned dating techniques that I didn't know. Now of course such a bit of information hardly goes as a credible source, but I often find that I get a hint about the direction in which I should look. 
 3) Of course, as a nice bonus, someone might happen to agree with me. 
 But, bottom line, it's because I like the challenge of analyzing an argument and finding its weak points. Oh, and I almost forgot: Those absolutely priceless sites like 
www.dhmo.org) and 
www.evolutionfacts.org) (I don't know why the latter link won't work). I laughed my butt off at both sites (although only the former was intended as a joke, the latter fills the role marvelously).