Yea, it is kind of a waste of time. But, I've had these type of conversations and correspondances as well. But, maybe something said will stick in their brain and later, maybe months or years later, something happens that triggers the remembrance of what was said and causes them to think, doubt or question things...maybe.
I had one JW who responded to something that I posted to on a news site. I had said in the course of my post that JWs believe in an imminent Armageddon where all wicked will be destroyed and only JWs will survive. We went back and forth a few times, debating certain points...I was trying to be the non-agressor and trying to let him know that I didn't "hate" JWs and that I felt they had the right to believe as they wish, but I was just trying to point out that they have some strange end times beliefs that need to be researched before anyone gets too involved with them. This guy ended up accusing me of slander and libel because of my statements over the Armageddon thing. I lost my patience with him at that point and quoted several Watchtowers and other publications that came right out and said that only JWs would survive Armageddon and I asked him to explain how I was slanderous when all I did was repeat what the Society themselves claim. He never replied after that.
So, as already stated, JWs will argue and debate until a nerve gets hit and they can't rationalize their position. It's kind of like Captain Kirk using logic ( I guess he learned it from Spock) to confuse a dangerous over-analytical, logical probe bent on destroying all imperfection. Kirk overloaded its circuits with its own logic and it ended up shutting down and self-destructing. JWs do the same thing...they just shut down when overloaded. They just don't go around saying "er-ror...er-ror...er-ror" before self destructing though.