Having done a couple of fam flights on the jumpseat I agree that they're certainly beneficial. It allows us controllers to see what goes on at certain points of the flight, the workloads that the pilots have and so on.
On the flip side, it's equally useful for pilots to visit ATC, preferably as many phases of the operation as possible. It would allow us to explain why things happen the way they do, why you end up on a heading when you can see nothing on TCAS (I'm looking at you, KLM pilots into Cardiff and Bristol
), why you might get step climbs etc and just generally increases the understanding.
NATS has finally started pushing fam flights a bit more, they've always been available, but for some reason they've been promoting them a bit more as part of our annual TRUCE training