While contracts are deterministic, there are lots of cases where a contract would need a random number (betting contracts being a prime example). One of the easiest ways to do this is to simple grab the hash from one of the last few block and hashing it with a salt to create a "random" number unique to this contract. For low-value contracts, this would work rather well, as miners wouldn't really have an incentive to try and game it, considering they could lose their mining reward. However, once Ethereum transitions to proof of stake, wouldn't this become a concern? It seems to be highly dependent on how proof of stake is finally implemented, but is preventing an actor from controlling the hash a current design goal. This may seem like a long ways off, but it kinda matters now so we can future-proof contracts.