On-line discussions are not meant to change the mind of the challenger. On the other hand, he lurkers' minds are changeable.
If a conservative exposes his position with facts and logic, he will be appealing to the large middle of undecided or uninformed lurkers.
I applaud any conservative who takes his time to engage a liberal when such possibility exists. I don't have the patience or the inclination to do it, but I admire the folks who do it.
You're right that online debates between a staunch Lib and Conservative never change their stances.
But like you said, I strongly believe there are dozens of on the fence lurkers who will say "Ah HA!" to a calm bit of logic. That's how I was converted, so to speak.
By kind people who pretty much said, "this is a healthy way to view life and the world"....been there ever since.