Why would you do that? For me it's important to note because premises are assertions, and conflicts of interest raise the likelihood that the premises have been deliberately constructed as to be false or misleading. The idea that I would throw out someone's argument because I've decided that it must be wrong in advance seems ludicrous.
Arguments should rise or fall on their own merits.
>But isn't that exactly what's happening in the infomercial case? And surely we shouldn't waste our (bounded, finite) time addressing, point-by-point, infomercial arguments?
Why exactly wouldn't we, and why would we assume that the claims that are made in infomercials are uniformly false? The reason I don't waste my time vetting the arguments in infomercials is because I'm not currently interested in a juicer, not because of a belief that people who make infomercials are liars, no matter how consistent their arguments or verifiable their premises.
>I typically do this with a 9/11 truther friend (also chem trains and luminati (sp?) and so on), and only consider his factoids non-fictional when I hear them from other sources as well.
I don't. The reason I ignore truthers is because their logic is consistently bad, and their premises are usually demonstrably false. Those reasons came after listening to their arguments, not before. There are plenty of questionable elements in the official narrative, too, but they're not in the same order of magnitude as the leaps of logic made by truthers.