Wait, you fool! You cannot do an hour-long usability session like that!
Well, the keyword is reductionism. UX research is discovering brain and behaviour. It's psychology. And a few psychologists have already been using crowd-source sites as sources of participants. [1, 2]
The overall results are quite promising: It is possible to undertake such experiments with a crowd-sourced experiment sample. There are, however, precautions to be taken, and it's only fair to pay participants a decent amount if only to ensure a low drop-out rate and faster participation.
So it's not cheaper but it is faster. One study  said, "Performing a full-sized replication of the Nosofsky et al.  data set in under 96 hours is revolutionary." For UX research, it shows a wonderful promise for particular questions as long as the experiment is designed well.
But I also want to make sure that I'm dealing with an ethical company. In my new role as a research lead for Vodafone, there is a reputation risk to the company. This means that I've been participating in some of these sites as a worker to check out conditions from within. A lot depends upon the micro-tasks conditions; but company also have their own attitudes to workers which was taken into account. We will not work with companies that argue about or unnecessarily delay payments to workers; or use petty reasoning to 'trick' workers out of their money.
To guard against reputational risk, we will engage only those companies that treat workers with at least some respect.
I don't expect this post to make any waves, at least not amongst the crowd-sourcing sites, because our work is fairly small potatoes for them. But we are a large company with expanding requirements, and it's always good to remember that those on the bottom can also, with a slight change in context, become the one who pays the piper.
References Paolacci, Chandler, Ipeirotis (2010) Running experiments on Amazon Mechanical Turk. Judgement and Decision Making, Vol. 5, no. 5.
 Crump MJC, McDonnell JV, Gureckis TM (2013) Evaluating Amazon's Mechanical Turk as a Tool for Experimental Behavioral Research. PLoS ONE 8(3): e57410. doi:10.1371/journal.pone.0057410