I wonder if they don't realize what "proper" form is. As a newbie to planks I've been searching for technique on how to do them. There are tons of videos on "how to do a plank". Some lacked basic information. Others gave great tips. All said when you start to lose form, quit. However, most did not specify what that meant. I found a video by a yoga instructor that demonstrated how to find one's most effective hand placement, very helpful. The directions given to me were very specific on what muscles to activate and said to place hands directly under the shoulders. The pilates video was more specific tan that. My PT assumed I knew how to do a side plank. He demonstrated it for me but did not have me do it with him. When I first tried them I wound up hurting my self. Then I found a video of a PT/trainer who was walking a client through a side plank. It was very helpful because it showed common mistakes (my mistakes) and how to correct them. I'm sure my PT would have helped me but I would have lost a week's worth of training.

My expectation when working with a trainer or PT is that they point out where my form is off and work with me until it is corrected. And yes, it is frustrating to put yourself up against others who do things sloppily and think they're stronger or better than you. I ski with friends like this. They have no technique but out weigh me by a lot so they can naturally go faster. I can't keep up unless I just let go and ski like them. I constantly work on my technique so it's frustrating for me to just be sloppy. My one satisfaction is that non groomed terrain (bumps, powder, trees) requires good technique so I handle it much better than they can.