Superman has a good point here about private messaging being just as effective. Sometimes it is actually MORE effective in getting a user's attention immediately. This all depends on the situation though.. It is kind of a hassle to PM somebody just to tell them "wrong section" or something like that, which is where I think the "backseat modding" or whatever you want to call it is perfectly fine if you are just genuinely wanting to offer help.
Personally, I try to communicate with users via PM as frequently as possible. If I see somebody who is repeatedly doing something, like posting short, unhelpful answers (spamming), I like to send them a friendly reminder so that they are aware of the rule rather than going around reporting, infracting, or creating a spammy post to tell them off.
Sometimes users just genuinely need a friendly reminder about certain things, or a gentle push in the right direction. This is why I think it should be entirely OK for users to help out in small ways like this when they see somebody who needs a reminder. With the backseat modding rule abolished, I think the forums function much better when we all help each other out a little bit, rather than relying solely on the moderators to pass along simple information, reminders, or point people to the right section.