There's nothing to stop people from setting up multiple accounts to run bounty campaigns either, so I'm not sure what some of the other posters are trying to say. Also, ICO investors often dump within days of the tokens/coins going live on exchanges, trying to cash in on the first initial surge, so trying to pin the blame on airdrops is a little naive.
Most airdrops are crap - with a few exceptions. Blue dropped 1000 Blue to folks a few months back, and they have performed very well. PolyMatch just wrapped their airdrop sign-up, but enforced KYC submissions, eliminating the risk of account spamming.
Like everything else in life, you have to be a little discerning on what you sign-up for. For one, any airdrop that asks you to make a donation to receive the drop, is probably a scam. Any airdrop/ICO using a bog standard ERC20 token, with no purpose other than "being used as a REAL currency" is going to be BS. That said, most airdrops require little to no effort, so it doesn't hurt to sign up for them. You never know which ones might eventually amount to something.
P.S. if anyone knows of any WAVES based airdrops, please add them, to this thread:
https://bitcointalksearch.org/topic/m.27939979