The biggest problem is that clients on those sites want you to build them facebook in two weeks for $300. People willing to work for this type of client are your run of the mill php spaghetti coders. They tend to attract the worst of both coders and clients.

I agree with jefflinwood's comment. Setting a price floor at a reasonable rate will weed out the "$300 for facebook" clients. I'm not sure, however, how you would weed out the bad coders.