Erik's Rule of Grammar Checkers

Grammar-checkers are certainly A Good Thing. But for the most part, they serve only to alert, not to educate. Because the people in need of grammatical guidance are unlikely to understand the advice being offered. Conversely, the people who understand the guidance likely don't need it.

So yes, grammar checkers are useful, but only in an inspect-quality-into-the-product sense. Writers with strong grammar will be alerted to their occasional copy-edit-style mistake. Writers with weak grammar, but still conscientious enough to want to eliminate outright mistakes, will at least see they have a problem, and engage in trial-and-error reformulation, to eliminate the mistake.