Airdrop is free coins so it's either a hit or miss thing,if the project become successful then it's a hit because you can get profit from it if not then it's one of the many scam that is rampant in many ICO's,a pump and dump scheme that is used by the team behind the project.
It's not really bad to join in airdrops as long as you don't invest money on it and don't just believe in news that advertise it's a good coins most probably it's a fake news.Conduct a background check on anything that is related to that coins if you're really keen on buying that as investment.