Because even if you can't convince the other guy of your point, you can still reach a middle ground. If you can get the other guy to agree to you, more power to you. The alternative is to beat each other up until the loser has to do what the winner says, and that doesn't work well, does it? Just don't bring in something inherently stupid to argue about, like religion. The religious have a right to believe whatever they want, and atheists have the right to not give a damn. Don't bring it up, and everyone's happy.
To everyone saying internet debates are worthless, that's not exactly true so long as the ones debating aren't idiots that go down to insulting each other. There are smart people on the net and in the world, it's just that the idiots stick out so much more. I actually haven't seen much raging stupidity here on the Escapist, have you?