I have a feeling that they just offer different gaming experiences. They both have advantages and disadvantages:
pbm allows for more time to flesh out your charater away from stat hunting, whereas live gaming give a more intense experience. I do both and I like both for those reasons. I interested what you guys think.
I haven't done play-by-mail since the 90s — even using email it's just too difficult to keep track of everything. Play-by-post on the other hand is a different matter. It's fairly easy to keep track of everything, you have an easily visible history of the game in chronological order, and you can really delve into roleplaying your character fully.
I enjoy live games for the social interaction and camaraderie, I enjoy PbPs for the shared storytelling.
dialogue is so slow in games other than real life; but I agree; both are good and have advantages.