I agree that freelancer sites have some problematic aspects, but I don't think there is anything wrong with the idea that you need to start small and build a reputation/profile. This applies to any bootstrapped business, not just when working via freelancer sites.
Unless you're suggesting that there's a meaningful difference in doing 40 hours of work for 5 bucks and working for free?
Edit: conceptually, it's nore like an unpaid internship. I know someone who got a highly paid job by first doing it for free for 3 months to prove his skills, as he was lacking formal certifications
Edit2: I honestly don't get why this is being downvoted. It's neither unfriendly nor offensive and it is relevant to the discussion, even if you might not like this aspect of reality.
And I know dozens of people who got burned by being talked into working for "exposure".
To me, that is a very different level of commitment by the company than working for "exposure" for someone who has no intention of hiring you.
People should get paid for work they do. If a task is worth $0 to a company, then they can do without it.
The whole "you're getting paid in valuable training/experience/exposure" thing is bunk, IMO.
The first thing I do when I write a game or software is send out advance copies for free.