Overall, these sites are still a merit-based system. Whether you are number one because of sales or creativity, you've earned it in some way. No one can fault you or your company for that. The only down-side that I've encountered, and confirmed with other users is; the language barriers that sometimes arise due to hiring outside your respective market/state/territory/country. It affects the work in some waya solely because the end-result is dependent on the initial client/creative interaction and communication.
I used a few of these sites years ago when they were fledgling beta startups. They were my first experience with a social network. They could be considered one of the first online-public social spaces. Some that come to mind; eLance ( beta orange ), bullhorn, smarterwork and guru.
Earning the work required the same tactics used by large companies, agencies, boutiques and freelancers.
The respective currencies of each user was the only true competition seen as 'harmful' to the industry. In fact, the long-term effect is far more valuable. Talent speaks many different languages.