For me, it's overwhelmingly just about spellchecking. I have not used JetPack or After The Deadline, but I do explore grammar and style plugins. I do find them valuable (two different tool-categories), but for one thing lots of my transgressions are part of my style.

It seems there is a high level of 'noise' in the feedback that such scripts have of offer. But there are gems, and I have blindspots, so this area is a worthwhile work-in-progress.