Airdrops were so popular among the crypto-community over the years. One of the key features of the airdrops is, it gives every participant a chance to get some free tokens by completing few tasks. Most importantly it is totally free of charge. But nowadays we frequently see a trend where the organizers of some of the airdrops ask to send a small fee prior to sending the tokens. In some cases, they put a condition saying that in order to withdraw tokens or any fund received from the sale of tokens, it is mandatory to take part in their ICOs. I would rather call such events as "Scam-drops" as they not only dilute the meaning of airdrops, but also make people to move away from future events due to lack of trust.
A distinction needs to be made between free airdrops and those which require a fee. I would never recommend those airdrops which are not free, because it goes against the very idea of airdrops. The main purpose is to distribute small amounts of cryptocurrency among the users, so that at least some of them gets interested in this particular cryptocurrency and start using it.