Having worked as a personal trainer in a commercial gym, and also having spoken with many experienced people in the fitness industry, this is my take on these type of issues.
I've discovered that many trainers and those in the training field go the "safe" route when it comes to training advice. This is why many trainers will stay away from heavy squats and deadlifts, and teach partial ROM on the bench. When you are training lots of different people and having to often give short answers to people in the gym all the time, it is a lot easier to give "safe" answers to keep people from getting hurt. There are liability and insurance issues involved, too.
For example, most of the trainers in my gym used partial ROM on bench, and I was one of the few that did full ROM. Since I want to do a powerlifting meet someday, I have to do full ROM. But when you train lots of different people -- everyone having different anatomical make-ups, injury histories, and level of exercise knowledge -- it is just easier sometimes to give the same advice all the time.
Also, you get people who come up to all the time in the gym that just want to know, "Hey, should I touch my chest when I bench, because someone told me you'll mess up your shoulder that way." These people usually don't know a whole lot about exercise and anatomy, and don't want a 5-10 minute answer. So instead of going over: (a) do you have a history of shoulder injuries or have had problems in the past (b) what is your bench style (elbows out or elbows tucked, arched back or not, etc.) (c) what is your main goal in training (looks, strength, sport performance) (d) what max bench would you be happy with? -- Then after going over all of these things, then you'd have to see for yourself their bench style, teach them to bench correctly, and then tell them that there is no problem for the majority of people with full ROM benching. But if you want to get to super-strength levels (300+ in bench) - then no matter what, you might get injured (torn pec, shoulder issues, etc.).
-- Instead of doing all of that - sometimes it's just easier to answer the question with, "It depends, but the safer route if you just want to get healthier and look good is just to do partial ROM, but most people with healthy shoulders will have no problem with full ROM, as long as you bench properly." Short 10 second answer they were looking for.
Same goes with deadlifts and squats -- you don't always know the person's injury history and they don't always give all the details. And unless you're a back expert, it's difficult to give answers to people without knowing in detail what you're dealing with. So most trainers just go the safe route and don't do heavy deadlifts and squats, if they even do them at all. It's just easier and saves them the scare of potentially hurting someone.
So I am a strong proponent of heavy squatting and deadlifting, full ROM bench, olympic lifting, and other things that many trainers don't like. But I sympathize with these trainers because I know why they hold the beliefs that they do -- it's just easier sometimes. And for the majority of people who just want to get healthy and look better through exercise, their advice is OK. So they can train 90% of the population just fine.
But if someone wants to bench 300 lbs. some day, or wants to gain 50 lbs. of LBM, or wants to be a professional athlete of some sort, then these trainers will probably not be able to help. But I can help someone with these goals. I mean, do you think they allow partial ROM at the NFL combine (I don't know for sure, but I'd be highly surprised if they allowed half-reps).
This is why I stopped training at a commercial gym.