When reviewing first or potentially bad posts, it's not horribly uncommon to find an answer that not only misses the point, but also has bad grammar or spelling, or is unclear. Being the sort of person that I am, I feel compelled to at least correct that grammar and spelling, even though the answer itself is bad in content. Is this harmful behavior? Benign? Actually helpful in some way to avoid future confusion from a reader?
It's never a bad thing to edit a post for grammar. In some cases, you may be wasting your own free time, but that's your choice.
In the typical case, you clean up the grammar on a bad answer and it gets deleted or ignored; no harm/no foul. However, that's not the only option. Other possibities:
- The OP might come back and see his answer marked NAA, and turn it into a comment, but use your cleaned up version.
- The OP might come back and try to improve the answer, and hopefully your grammatically improved version will remain intact.
- The OP might come back and see your cleanup and learn something about English grammar they did not know, which will help them in the future.
(Yes, I'm an optimistic person.)
This subject just came up on meta.so today as well.
I haven't seen full confirmation of this, but this answer mentions an important point if it's true. According to their (not a mod/staff) statement, a grammar edit could bump the question out of the VLQ/Deletion review queue. I've never noticed this to be the behavior, but it generally makes sense.
Posts in the VLQ review queue are immediately removed from the queue when the question is edited. You could have a post with 5 "Recommend" delete votes, but someone with 1 rep or an anonymous user (with the help of 3 reviewers) can block deletion simply by editing the question. It basically made the effort of the VLQ reviewers worthless as those recommend delete votes are gone. And since this is done without any fanfare, no one knows it happened.