It depends. With a lot of topics, confirmation bias means you can argue with a wall and get a more productive discussion out it than you would arguing with posters up here.
Still, even if you can't convince those you're arguing with to your POV or they can't convince you, if one of you can at least understand the other's stance, I'd say it wasn't pointless. If someone walks away more knowledgable on an issue even without changing their stance on things, I'd say it wasn't a waste. You can't effective argue against a certain stance on a given topic if you don't even know what that stance entails, so having a more nuanced view of a topic makes you able to more effectively make your own arguments.