wow, this is really interesting to hear; See, I thought that nearly all work on these type sites (my experience has been with e-lance, many years ago) had mechanisms for paying people through the site, but from what I saw, only the first transaction was done that way. After that, the contractor and contractee worked directly, so it's interesting to read that some people found enough value in the structure provided by the site to continue using it after meeting a person.
Well, some people just use it for payment later on if they don't want the hassle of wire transfers and such.
For RAC, they used to charge 10% of the transaction value so it could build up. My experience was that most people contacted me off site after our first project but sometimes still used RAC for payment because it was easier for them(I can't use paypal). Although they were nice enough to take on the 10% charge.