My largest issue with this idea... Since when have the "rules" actually mattered when it comes to writing? I enjoy playing with words and do not want a pile of squiggles telling me I have to comply with a rule that destroys the rhythm of what I'm working on. "My voice", as others have often called the way an individual writer ... writes, should be unique to me and not some automate clone of grammar rules.
And you're free to ignore any suggestions such a system would make. You'd even be free to — gasp! — turn such a system "off" if you wished, I'd imagine. Free to not use it at all. (I know, it's a radical concept, the idea of not using a provided feature because you don't need it, rather than having it not available at all to anyone else who might actually need it or want it.) Those of us who do want to follow basic grammar rules, however, and who do delight in working within the established parameters of the English language, would like to have Scrivener support some measure of telling us when we get things wrong or right, however. I don't think this is too far of a bridge to cross for a piece of software that bills itself as being "y'know, for writers."
However, when I think about prose editing, I recall one not so good story that once happened with my friend. He composed a few funny short stories and read them on one of the literature evenings we had in our town.