In all probability, anyone setting out to actually defeat these algorithms could easily do it with a couple of hundred repetitions of the same sentence.

Or, if it's slightly cleverer than that, certainly you could defeat it by producing a single, perfect-length, stylistically fantastic essay... which would be regurgitated word-for-word regardless of the subject matter.

I think, mind you, that software does have a place in analyzing student essays. If I could scan in an essay and have it spit out a word count, highlight any spelling errors or potentially problematic turns of phrase, and [most importantly] analyze for plagiarism, that would be valuable.