We're encouraged at SE to edit each other's posts to improve them: The edit button is right there; all text is Creative Commons–licensed; we get badges for doing a lot of editing; the review system includes an "Improve" button. Everywhere, the site gives us permission to fix each other's mistakes.
So the way to handle poor grammar is to edit it. Leaving comments is unnecessary – no-one would suddenly majorly improve their grammar just because someone says to, because anyone who would be receptive to the message is already paying as much attention to good grammar as they are able. On the flip side, someone who doesn't improve their grammar when they see their post edited is never going to improve it – they're either unable or unwilling to change their grammar skills. A comment is just going to be unconstructive.
The whole point of SE is that the collective wisdom and skill of the community is greater than any one person could have, so it gives us tools to harness everyone's abilities to make the every page as high-quality as it can be. That includes the quality of the spelling, grammar, and formatting in questions and answers.