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Personally, I would not find it a waste to spend 20 minutes reading and providing feedback. I think it would hone my own writing abilities. Also, I wouldn't mind if I got bad advice. Live and learn.

I picture this as a good way to a) get feedback, b) improve your writing skills, and c) connect with other developers. Mechanical turk can provide a, might help with b, but in no way can provide c.

That said, it's definitely a good choice if you just want feedback on your copy. An even better choice is a/b testing.