It makes me sad that so much of the work available on Mechanical Turk is poorly designed, and that workers have little recourse to bad Requesters.
If min wage is say $7.25 an hour, which is what you aim to pay turks... but then you end up throwing away 2/3 of your work (say have 3 turks do each piece to validate they did it right).... why not hire 1 local person for $15 an hour? Decent wage, less total money (15 vs $22 an hour), and likely better results.
I didn't see an answer to this ... so I'll take a stab at it.
One advantage of using a turk is it's on an as needed basis. Hire someone for 1 hour, 1 week, or even 1 month of what used to be called "piecework".
That's quite different than hiring a person locally. Where can you find local people to do piecework? It's quite likely the task or tasks envisioned don't constitute full time or even part time employment. They're just one-time tasks.
Maybe temp agencies still supply people for jobs like this? But you're limiting yourself to a small pool of people unless you're in a big metropolitan area or unless there's a University with a large group of hungry students nearby.