Yes, it is good idea. Why not? If the person cares about being useful he will either delete it or improve it. You can also flag the question/answer for being low quality so the reviewers will leave the proper comment.
The downside of it it is only it is the time-consuming operation. You may find out sometimes a lot of posts are incomplete/wrong so taking care about all of them is hard. Sometimes just downvote is enough. You have the choice to leave the comment or not. And unless you were wrong in several times you will not get message from moderators or those you are trying to help (you are trying to help them, right?).
Also, there is a scriptscript that saves me a much time leaving comments.
