I agree with you completely, there is no point discussing something that does not exist.
But how do we know what is real what is not? Do we actually know who we are and where we are...?
http://www.illuzia.net/en/videos/clips/reality
http://www.perceivingreality.com
All the best.
For the same reason that people try to prove that Clark Kent / Superman is not a real person - personal self-insecurity.
The fact is, as learned in 1st year college psych-101, that one does NOT try to disprove something he KNOWS not to be real. In other words, you don't try to disprove the existence of CK/Superman since you KNOW it is just a made up character.
But people trying to disprove the existence of God tells me that they have deep inner doubts whether God really doesn't exist.
The best argument is when you are the Devils advocate. Always argue to create thought, promote self awareness and mostly just to aggravate the person you are arguing with.