I don't join because I have no use for trout flies, or steelhead flies, as we don't have trout or steelhead here. And as far as who does particpates, when you make statements that the flies you rec'd "sucked", I would imagine that it would intimidate a new tyer, to the point that they would be leery of getting into a swap.
I do agree that if you DO participate, you should make sure that you can make the deadline prior to signing up, so everyone will get the flies on time and not have to send repeated e-mails out explaining why nothing has been sent out yet.
And regarding my first statement, I teach a tying class once a month at a local shop, been doing it for more than 2 years, and we have never tied a freshwater fly in the class, due to an extreme lack of interest in freshwater fly fishng. Not saying no one fishes freshwater, just nobody wants to tie those flies.