In other words, I believe (based on what I've seen so far) that main reason why someone would apply with multiple accounts in the same signature campaign is to join with all of them, and not just to increase chances of being accepted and then join with only one account and withdraw the rest.

If members generally don't have intention to break campaign rules and they just want to join different signature campaigns on different accounts, why its so rare to see someone making it public that he has another account and that he joins other signature campaigns with it? There might be more, but I know about only 2 cases of that.