Some people spam such services in a bid to multiply the little payments attached to it, this could take a lot of time, creating different email addresses and accounts and when compared to putting that time in productive work it still falls short. There's the other risk of exposure to scams, in a bid on o get free stuffs people ignore actual knowledge about what they are doing and risk falling victim to cheap scams, such as; send 0.01BTC to claim 0.1BTC instantly.
Productive effort, even with no immediate gain will always be more rewarding in the long run.