Ace
First, like most of the teachers I know, I grade the work not the student. And yes, one can reward "effort," in the form of a higher class participation grade.
But there are agreed upon standards for an A paper, a B paper, etc. We even do "norming" sessions to make sure we're all using the same standards and criteria, not just for papers, but for exams.
In terms of writing for such sites, I'd think poorly of someone who did--and that writer is not going to be making enough money to make it worthwhile. Remember that the sites don't usually want an A paper; they want papers in the C and high B range, on the theory that they are less easy to "spot"--and yes, I do know how they work. Intimately, and, though I shouldn't have to say it, for a good cause--to shut one down.
The sites that charge a "research" fee give students with money a better chance, while the sites that are "free" not only have crappy essays (which will show up when we google phrases from a student's paper) but again give students with the time and ready access to the 'net an advantage.
Finally, I'm really not interested in what some anonymous writer for hire thinks, or how that person writes; I'm interested in how my students think and in their writing. They can improve their own writing. I'm not sure they can do much for an anonymous hack.