I have showdead on, and almost every single flagged post I've seen definitely deserves it. Every time it wasn't "deserved", the person simply took an overly aggressive tone for no real reason.
In short, I've never seen somebody flagged simply for having the wrong opinion. Even controversial opinions tend to stay unflagged, unless they're incredibly dangerous or unhinged.
I've seen a few dead posts where there was an innocent misunderstanding or wrong assumption. In those cases it would have been beneficial to keep the post visible and post a response, so that readers with similarly mistaken assumptions could have seen a correction. Small minority of dead posts though. They can be vouched for actually but of course this is unlikely to happen.
I agree that most dead posts would be a distraction and good to have been kept out.
In short, I've never seen somebody flagged simply for having the wrong opinion. Even controversial opinions tend to stay unflagged, unless they're incredibly dangerous or unhinged.