My question was for /dev/random. The main problem RdRand solves is quantity: obtaining a lot of random bits per second. Even if they were produced in a way that somebody knows possible weaknesses, mixing them with something cryptographically strong where we control the seed we'd preserve quite a high throughput. I know that there is /dev/urandom which can often be good enough, but I know that too much applications in fact prefer to use /dev/random so making /dev/random robust has sense.

I see Ted Ts'o commented too, and as I understand, having RdRand is still much better compared to having the platforms without it. There's a lot more to care about than is RdRand "perfect" and once you have something like RdRand you can use it safely enough, compared to not having anything.