As a seller, there is nothing which I can’t do via the Fiverr messaging service once I am engaged with a client. What really annoys me, is when the best clients in the world try to send me their contact details to communicate outside of Fiverr. I mean this can risk me loosing my account and simply isn’t necessary.
As for sites which do allow communication outside of their given platform, why are you moaning here about Fiverr’s shortcomings and not already using them instead? Is it because you probably won’t be able to get what you need for $5 by any chance?