That would be a cynical view of it, sure. I do consulting work, 90%+ of it out of sight. I could technically hire some offshore help to do it for me, but it turns out that the reason people pay me what they do is the insight and knowledge that I bring to the work. The exact same principal applies here.
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.
Maybe I'm wrong, but I think they've changed their front page over the course of the day (perhaps in response to these comments).
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.