Airdrops are for free but you still have to invest your time, which has a value, so you should anyway chose the right projects.
I participated in couple airdrops, few doesn`t worth anything, few I didn`t even receive, couple turns out to be a scam projects. Time is most valuable things you are right about that, and many of this airdrops are just waste of time and energy. Now I check more closely airdrop coin, if I don`t see something good I don`t participate. Its a waste of time to be a airdrop hunter I think, maybe someone have a success but I would like to hear how much in percents if its possible, from how many airdrops someone earned specific amount of money.