Tools like Fiverr, Mechanical Turk, etc are an amazing way to create positive trade with low-income nations. They get a living wage, we get cheap labor, everyone wins.
They are an abysmal way to run a sustainable first-world economy, due to all the problems listed in the many comments above.
But don't let the shittiness of a gig economy in the US, EU, and other prosperous areas overshadow the value they have in allowing poor areas of the world an instant economic advantage. The Internet has allowed us a way to provide aid without creating beggers, to create a cash flow where value is moving in the both directions, and to allow for economic success in developing nations without sweatshops and mines, without employers siphoning off most of the wealth, or warlords stealing the crops.
Five dollars for an hour of work is shitty here, but when five dollars can be a days wage (or a weeks) in many places it's amazing. If they can get Internet access (and that's a big if...) then it opens up a huge economic opportunity for many of the poorest nations. This kind of opportunity is why Google projects to get the Internet out to rural Africa, India, and South America are so vital.
So yes, it sucks for us here. It should be fought. But the idea itself has merit, it's just where it's being applied that is inappropriate.