I have too, but only after discovering rationalism. Original line was meant to apply to general internet.
And in the broadest strokes that statement is objectively false - I was deconverted from christianity via online argument. However the arguing was repeated confrontations over several weeks, and then the deconversion didn't take place until months later - after I'd completely lost touch with my opponent. I've never been able to contact him and thank him properly.
But for the average person, any single argument session will not noticeably change their position. I feel that adding any caveats to the original statement makes the emotional weight swing far away enough from 0 that the caveat makes the statement less true in practice. Similar to how you are better off saying "You have no chance of winning the lottery, ever" than you are saying "There is an infinitesimal chance you'll win the lottery". One cannot activate few enough neurons to properly convey the chances of winning when thinking about the chances of winning, and thus accepting "You'll never win" is closer to the truth when run on human brains.
My experience of there being plenty of people online who will change their mind isn't limited to only rationalist circles, though it is mostly limited to circles with people of above average intelligence. Perhaps if you are actually talking about the average Internet user, it's justified to make the kind of assertion that you were making, but I wouldn't expect most LWers to hang out in the kinds of online circles that are dominated by average Internet users.
I wont be the only one here who "wastes time" arguing about things they care about online (note: I am referring to web forums and things like subreddits, I am not including Less Wrong whose dynamic is completely different). It seems like something that is worth optimising in some direction.
The theory behind it is that one should expose themselves to counter-arguments allowing their claims to be attacked so they that have a chance to substantiate them or reject them upon realising they are mistaken.
In practice they generally follow a pattern that starts with people pointing out what they believe are mistakes then ignoring or intentionally misunderstanding the other party when he refutes or backs up claims.. and ends up with insults, patronising sarcastic remarks and nobody changing their mind about anything.
I don't particularly care about changing other peoples minds to make them agree with me (well, it would be great but I think it's practically impossible) so one thing I would like is for both people to at least end up feeling good.
So I'm interested in three things: Do other LWers recognize this pattern now that I have mentioned it? What decision did those that were already aware of it make, in order to optimise this activity?