The main reason I say is because I think a lot of people, based on the amateur writing I've seen, don't need a fancier way of arranging words so much as they need to focus on 1) the quality of their sentences and 2) how one event drives another in their plots. I worry especially regarding point 2) that Scrivener lets people work in parallel when they should be working in serial, with one event driving another organically. Too much amateur writing I see is, for lack of a better term, plotless: meandering around feelings, or random encounters, or designed to show how deep the author is—instead of telling a story.
Scrivener will help with some things, as I've written elsewhere, but I'm not sure it's really enough for the vast majority of what writers and would-be writers are working on.