You're not really doing an artist a favor by under paying them. The real reason people go to sites like freelancer.com is to find cheap labor. Artists have to under bid one another in order to be considered for the job. As a result they have to take on more projects to make up for the money they lost under bidding. Artists are over worked, over stressed, they don't get enough sleep, and they don't have time to produce quality work. They can't spend a lot of time on your project because they can't afford to. That's what I mean by unhealthy. It's not good for anyone.
It's also untrue that artists need paid work to build a nice portfolio. We can create art without it. Nearly all artists have work in their portfolio that they weren't paid to create.
jchamplain - Your argument doesn't make any sense. It's up to the artists to decide on whether or not they want to pick up jobs. If they feel they are getting underpaid then why would they be on freelancer.com or anything else to begin with?
A site like that simply gives them more options. If they're in a 3rd world country for instance, they probably don't have much choice in their local market, whereas on something like freelancer they'd have the globe within their reach. Additionally foreign currency may be worth more to them, due to exchange rates.