I think it's a half truth to say that -fragging doesn't mean anything.
+/- fragging a post is basically saying "I like/dislike this." If a post gets more than one or two -frags, that essentially speaks for the community as saying "we don't like this post." I don't believe that the +/- Frag system exists just for no reason.
There are some questions like "What's your sensitivity?" that pop up from time to time. Even if the regulars here know that the question is a bit misguided, most of us have probably asked it before.
If we want more people to love TF2, let's not give them shit for asking questions that we just assume everyone should know.