I always stay away from them, the rollover requirements are usually way to big.
They use them to lure you in, get you playing, preferably addicted and in 99% of the cases they do not lose any money from it.
You can try out games with play credits as well or if they have a faucet, you can use that.
Bonuses without rollover requirements are pretty good, but you don't see them very often.
Bonuses without any rollover requirement do usually can be seen or can be acquired when a gambling site is on launching date but same as you said its really a rare case.They are building business not a charity. They might give free money but wont be that much. Bonuses and promotions are just really being made for luring in people and as expected wager requirements are too high which most gamblers would able to fail to reach out in the end.
Some casinos actually give away quite a lot of money, in terms of faucet bonuses.
I wonder how much Crypto-Games actually spends on their faucet, I personally get 26 x 0.00001590 BTC per day.
For all the members they have, that must be quite a lot. Also with the rain they give away in chat.
Their bonuses never have rollover requirements btw.