but not all bounty managers need this method. Every project has its own rules, if Mnager Bounty doesn't need Proof of Authentication then you don't have to. It's simple, if you don't want to get a negative Trust or be perceived as spam, then you should carefully read every project rule you want to follow.
I don't think so, it is not about the bounty managers who need this method. This is only for the safety of every members to avoid bounty cheaters. Most of the time, the account of other members here are being used to apply in different bounty campaigns even without their knowledge. To avoid this, POA should be done and I think this is also important.