Provably fair is not a must, it is much more than that - it is industry standard. No one will gonna take seriously gambling site without this system implemented.
It is not an industry standard yet.There are lot of sites running their own scripts but of course they're verifiable.Its not about taking a gambling site seriously ,cross verifying the last server and hashes are equally important.
But there are cases when you simply can't have Probably Fair system in place - for example PF system won't work with live games like Roulette or Black Jack.
Why not ? One can always have a rigged Roulette with results always opposite to what bets user places.Black jack ,to an extent its not necessary but the cards can be manipulated ,you never know.