[GuthL]

While you are right for most applications, it would not work for gambling or block selection in the case of proof of stake. Let say I'm flipping a coin (based on the last bit of the block hash) with you. You already locked the funds on the contract, waiting for me to commit to. I can wait until I'm the block proposer to push a block where I'm included. Even if we defined ahead of time how many rounds of hashing, I still have access to that information ahead of time and can decide to participate in the bet. Therefore, it is not secure randomness. (I won't get into commit and wait which could solve it but they are a pain in any case)

[tfha]

No need for commit reveal, just do a relative offset for picking the rng. So after you place a bet, 6 blocks later is the block that decides if you succeed or fail.

You can use simple techniques like this to make most use cases secure.