You have point on that, but bounty campaign is really helping to promote the project by retweeting a tweet or simply share facebook post sure will help a lot, and you have no idea how much people joining in it. In other hand, yes airdrop really attract more people like you say but its only spammers who dang share his refferal link everywhere. But not all airdrops need that affiliate or something like it. Some airdrops really just giving away his token to user and build a strong community, ever heard of blue token ? its 100% airdrops.