"Useful" is a huge stretch. I tried the game "egg, inc" this weekend and one of the mechanics forces you to view adds for other apps, and it's all addiction mechanics with an indecent 'skin' on them.

For example, there is one bejeweled-like game where there is a king who is being tortured, and you must make matches at speed or the spikes are driven into him, or he is forced to pee himself. There is another 'match two' like game where your reward is to take a female character and shave her head so she acts ashamed, and then dress her in streetwalker clothing.

Ultimately, Apple (and Google) have incentivized developers to make terrible games full of ads. Perhaps that is a larger problem that requires a broader, more systemic solution.