Would it be more sanitized?
Maybe there were less pointless posts. But I wouldn't take it for granted.
Would it be less filled with spams?
Not sure. On one hand if you take away the incentive to frequently post, there would be less meaningless topics and replies, which reproduce themselves, but on the other hand, you would also take away legitimate users who do help (and don't have the time to do it voluntarily obviously). Check topics from 2012-14, when there weren't signature campaigns, and tell me if you'd rather have that. I wouldn't.