...and besides, it’s good practice for when you want to argue pot laws with a subscriber to High Times.
You can’t argue points or faith/belief. The more the person who believes argues, the more he or she feels they are strengthing their faith. Very often faith is strengthened in or through conflict.
Whether it's pointless depends on the intended audience. Many online debates -- or for that matter, most in-person public debates -- are playing to the lurkers. Get both sides out there, and no matter which side (if either) a listener chooses, at least they know that no position is uncontroversial or axiomatic.
No matter what folks believe, I think they make better decisions and judgments if they've at least heard, and preferably considered, both (or more than two) sides. I'm not just playing devil's advocate -- I believe everything I wrote. But i also don't believe it's pointless to voice my opinion, even if there's a zero percent chance of convincing the person I'm addressing most directly.