Airdrop as a promotion for ico? i don't think that's a good idea because most of the one that join airdrop only search for free tokens and won't invest at the ico, the team also has to manage the airdrop and that's not an easy task to do, not to mention that the one that joined the airdrop can attack the project if there's some delay or they don't get the token. It's not worth ruining the project image for that, better to search another way to attract possible investors.