I've done this at times to a well-written argument that I disagree with. Could be just basic fairness, hoping to attract somebody to counter-argue, or possibly to help it rise above more poorly-reasoned arguments for the same thing, or even to make my own counter-argument post more visible.
Sometimes I do. If I don't, it's probably because I just don't have the time or energy to write a well-reasoned counter-argument at the moment. Or maybe because I don't know enough, and don't feel like doing the research to support what I think. Getting sucked into internet arguments at work or while working on my own projects is just terrible for productivity and focus.
