'Grammatic' is a feature in Word that I have really been disappointed to find isn't really available in any other word processor. It really is a must-have for writing anything solo, for school or for work. I'm just glad Word is available on both platforms. Otherwise I'd be screwed.
For me it's extremely convenient to just set Word to watch for little details that pertain to writing in APA format. Like, use of first person, contractions, spacing after sentence, writing style, readability, position of punctuation in quotes, etc.
I guess those aren't all necessarily conventions of grammar so much as writing style/format. But it's a tool I find necessary, as a student, desk jockey, blogger, etc. Overall I would really prefer writing in Pages, it feels more natural, and it is extremely handy that it works more like a publishing-layout tool vs. just word processing. If it had 'Grammatic'/writing style editing it would be the only tool I would need. As is it, I type everything in Word to help keep my style and grammar uniform, and consistent across everything I write.