I think there are reasons to discuss religion on-line besides those I listed earlier. First, I think there is no point to an argument where one cannot be convinced that one is wrong. By this I don't mean that you cannot be convinced that you are right; but you have to take the argument seriously, and risk refutation.
Sometimes argument is called for when someone makes an argument that is utter nonsense. That's a point when I'm willing to cross denominational lines. When I'm doing that I try to stay within the confines of the denominational context in which the argument appeared. This is a hard stunt to pull off, I admit, and I do slip; some people have tried to argue that it's intrisically impossible and that I always argue as an Episcopalian. That's too strong a claim. When it comes to matters of historical fact, for instance, denomination doesn't matter.
Likewise, when people makes erroneous assertions about Anglicanism and the Episcopal Church, I feel no qualms about correcting them. Here I must qualify the notion of "error"; I don't mean disagreements between theologies, but caims about what Episcopalians are supposed to believe and the like.
I have no problems with these because getting your facts straight is intrinsically beneficial. Entering the subjective arena of theological disputation requires a different rationale. Here, I think the only reasonable purpose is to refine the quality of your own arguments by putting them to the test of argument. If you don't think they are subject to discussion (and possible refutation), then why bother? When you make inarguable arguments, you aren't really listening; listening requires understanding, and understanding risks agreement.