The fundamental issue with these sites is that they attract a global labor pool to openly compete with one another for extremely price sensitive clients without the protection of artificial price controls.
A valuable, related problem to solve is "how do I find a few people I could rationally believe will help me get positive return on my investment." And, so far as I know, no one has figured out how to convey the information we humans need to arrive at that belief or trust via web pages (or printed yellow ones)—notions like "we only register l33t with GitHub cred" and Ebay-esque star ratings not withstanding.
True, but why let that be an issue? No one's being forced to be either an 'extremely price sensitive client' nor a freelancer competing to work for one.
Sums up my experiences with freelancers pretty well. Although we all have to start somewhere...
Edit: My experiences being that the cheaper ones are monkeys, not all freelancers ;)