I don't think a "helpful" button would be useful.

Maybe a "like" button like Facebook would be nice. You can only "like" a post, but it is not a numerical rating of the post. What would be different is that it doesn't display who liked it.

In theory it would say "4 users have liked this post"... instead of on facebook: "ABC, DEF, and 4 other people like this"

It would be that way to not copy the way that other sites have their system and make it be more about whether you like it or not versus a numerical rating (which could be negative).