Repeated PD is very different. The whole point of a cartel is that everyone inside is incentivized not to cheat, and it works as long as they agree on what's fair. The problem for cartels is people not in the cartel.
So what? Anyway the more important difference is it requires both computers to be on the same LAN. The main point of AirDrop is you can share between two devices with or without LAN, with or without internet.
I only use it for sending a bunch of photos that wouldn't go so well over iMessage, usually on vacation. And sometimes that vacation involves not having cell service. Otherwise I'll usually message a single photo to someone because AirDrop is finicky.