Also, I should mention that if someone wanted to cheat, they wouldn't plagiarize; they would simply get someone to write a paper for them. This service is far, far more common that you would believe.

When I was in school students did plagiarize a lot. It went out of style as software was introduced. I distinctly remember the panic'ed buzz as students discovered the teacher (or professor) was using detection software. The services you're talking about are common precisely because of plagiarism detection software.

I'm not disagreeing with your points about morality of it though, just the efficacy.