The whole problem is there is no set of rules for having an argument. The likes of you (I suspect) and I like to discuss what we would call the point in question. We expect that we have a viewpoint and the (for want of a better word) opponent has a viewpoint. We make our arguments for and against and if someone is right or someone is wrong it soon becomes obvious which is which.
To some other people it appears the point in question is merely the starting point. The link to the original argument can become increasingly tenuous until the likes of you and I suddenly discover we're discussing something completely different.
I have a theory that this tactic of sometimes subtle, sometimes extreme, change of subject is a defensive tactic used when someone feels they are on shakey ground and likely to loose the argument. Wheras the honorable path is to say "Fair enough old chap, I think you're right on this one" they can't do this and their contention would be that as long as they are still saying something the argument is not over and thus not lost.

If this has the feeling of truth to you Iomaca you are right to drop out of a discussion that you feel has gone off the rails. Because you too must feel the same frustration I have with people who don't argue by the rules. But don't give up grasshopper. If you see something that you dissagree with - say so. If people like you and I don't comment, the forum - and yes Goddam it the world - becomes a poorer place. It takes strength to say your piece, know you are right and then shut up but until they put an icon on here which says "Smug smile" we just have to grit our teeth and be strong. Content yourself with the thought that if by standing up for right, truth and the Australian way (of arguing) you save one poor punter from making one poor bet..... yadee, yadee, yada... Even I've stopped reading.

Extract from page 217 of "The Thoughts of Chairman Kenny"