It's a good deal in some ways. I've heard some people have gotten in the neighborhood of $15,000 to allow Graphicstock to sell their work. For some people, they'd never make that much at most other sites over the course of several years, so it could be a good deal.
I was negotiating with them to sell my work but it fell through because I don't have a big enough portfolio. They thought I had over 1,000 vectors based on my Shutterstock portfolio, but I told them I really have about half that many because of the separate vector/jpg files at SS.
If the price was right, I might have done it. I don't see the harm in it. Sure if they get big enough you might be competing with yourself, having images at Graphicstock and still trying to sell the same images at other places. But that's not going to happen tomorrow. When you cut a deal with them they get your portfolio as it is at the time of the deal. Anything else after that is still exclusively yours. So even if they get big and start to be real competition for other companies, that will take time, maybe years. By then, all of the work you sold them is old stuff.