Discussing subjects like DNA and geology with someone who's unfamiliar with scientific theory is amusing, but perhaps educational. Engaging someone you know is an inflexible fundie in debate over such emotionally-charged issues over and over again is a sign that neither of you know when it's time to quit.
I understand your point about the very low likelihood of someone changing their mind on these social issues, but I like to believe that some positive change has come of it.
There was a thread started by someone who vehemently stated for 100 posts or so that homosexuality was wrong because sex without the possibility of procreation was immoral. When countered with the example of infertile heterosexual couples, he admitted he had a vasectomy - and I read into his posts that he began to rethink his previously immobile stance upon the comparison.
It seems to me the chances of educating an "inflexible fundie" on abiogenesis or evolution is about the same as educating them on homosexuality. That doesn't mean it isn't worth trying.
Importantly, Lam repeatedly tries to maintain his threads as Biblically-oriented, so at least the originator is attempting to maintain a Biblical context. Likewise, homosexuality seems to intuitively fly in the face of evolution, and thus the biology of homosexuality has produced some rather interesting (and educational) discussion.