Its always depend on the project I saw many people earn in airdrop by referring people and become worthy after the airdrop is finish . and there is also an airdrop that become worthless after the distribution .

Mostly of that airdrop you will receive your become the only questions is how much is the value of that currency after the airdrop. It's free so it's ok even the coins will not have  value you will only wasted effort for promoting it .