When I'm in a scene I'm there to play, and personally I can't stand poses that take 15+ minutes to write. I know a lot of people don't play that way because of RL things going on, but it is what it is.
I could see a mu* integrating PbP more formally (it's already kind of there with +jobs/+bboards/@mail) which maybe would satisfy slower RPers. But I suspect those people would protest they're not actually that slow.