After years of only working out in my basement gym, I joined a commerical gym so I could use the machines. Hamstring curls and quad extensions have been the key to rehabbing both of my knees after beating the crap out of them training for a half marathon. I couldn't go up or down stairs without sharp shooting pains. I was doing plenty of RDL's , squats, deadlifts, cleans, etc., but isolating those muscles has made a big difference. Likewise, adding in a bunch of shoulder and upper back work has mitigated most of my shoulder pain and range of motion problems. Bodybuilding for the win. :)
I know a bunch of people who SWEAR that you cannot rehabilitate knees without a knee flexion/extension bench. I've been fortunate that knees have not been my problem--yet and hopefully ever--but I'm not surprised that you had success using them. While obviously, if people want to compete in strength sports a certain amount of specificity is king, most people who be better served by doing a wider variety of exercises instead of just #benchsquatdead chimpouts. And when I say "better served" I mean that they would both look and feel better, and as a consequence, possibly even become stronger in the long run.
I know that my shoulder absolutely requires variety from me, and a lot of people I have heard mention similar experiences. Pressing at all angles, sure, but also rowing, curling, front, lateral and rear raises. I believe it was BD who talked about curls being a major component of getting his shoulders in order.