Good grammar rule software looks like someone effortlessly, but gently, correcting small mistakes: "yes, but this word goes first here."
Bad grammar rule software looks like a non-native speaker making fossilized errors: "No, it's 'He not have it'"
Conversation
Having seen this, I'm not really sure if grammar-correcting software has much real utility for the user.
If it corrects some errors, but introduces or reinforces certain error patterns, it will make non-savvy users struggle more in the long run.
1