I'm pretty sure I've suggested this before, if not here, then on IRC. However, at a low number like 5 reports, I or someone else could easily arrange any thread to be hidden.
Pretty much any system can be abused in one way or another. At that point, it would be up to the mods to handle it appropriately. As for the fear of reporting a thread and having the mod disagree, obviously infractions or other similar actions should only be given if it's blatantly obvious that the post was flagged abusively. And also note that it would take several people flagging it for the post to be hidden, which would make it even more obvious, especially if they see that it's the same person using multiple accounts to do so.