I was helping someone edit a book, and that syntax checker came in very handy. There were indeed many false positives, but it provided a much smaller search space for my eyes than 'every word on every page'. (I did read every word, but the ones with the green lines under them got extra focus that was not going to be available for 200+ pages.)
I also love how the very people who can spend tens of hours on configuring vim, emacs, bash, zsh or their IDE can't be bothered to spend 5 seconds disabling the Office spellchecker if it bothers them so much :)
I'm glad that feature worked for somebody--that's definitely a different use case. For smaller documents it's more effective just to reread the doc carefully.
Spell checks on the other hand are invaluable, especially since you can extend the dictionary to cover new words like "deserialization."
