Although in general flagging a post may be often followed by removing the post, flagging is not *request* to remove it, at least that is not my intention. If I have the feeling a post is bad but "can be saved", I leave a comment. I noticed in many cases the post stays for a while, giving the poster the opportunity to act, even is the flag is marked "helpful". I think that is the right procedure to prevent being rude. 

So in my opinion, yes, if you feel the post is sincere, which it is in many cases, you can very well leave a comment, but you should still flag it (with care), if it is a bad one. The moderators take care of the policy what to do whith it. 

If you don't flag a bad post, who is looking after what is happening with it?