There was a service years back that did something similar via the phone -- you could register as an expert of something and people would call and pay them for time with you. It is a good idea, fundamentally, though voice was a horrendous medium for it.
I can see something like this (not necessarily this implementation with their ridiculous psuedo-currency) having applicability for core developers of widely used projects (Apache, nginx, Linux, Cyanogen, etc). If they gave useless replies the jig would be up.
Aside from the technical, in the gaming community lots of people like to email Gabe Newell about Half Life 3, for funny comments, etc, usually to cash in for karma on Reddit. It would make it more interesting if he could have something like this with proceeds donated to charity.
When I first read it, it looked like a way for advertisers to reach potential buyers. If that's the case, then someone on oDesk could easily respond pretending to be interested in a product just as well as I could.
Envisioned more broadly as a way to pay for the time someone spends responding to email, it would be hard to outsource my role in email correspondance.