Proper form is an issue that I've seen beginners and advance fitness enthusiast both struggle with. For beginners their lack of knowledge is what leads to their poor form. They haven't had any one school them about the importance of form yet. If they did, they would learn that the correct form will prevent them from injuries and allow them to get the most out of the exercise by properly targeting the muscle intended to use.
What surprises me are the lifters who appear to be veterans in the world of fitness. Squatting with too much weight, curling by swinging arms, even walking on the treadmill while holding on to the bar in front (which is just a pet peeve of mine). Squatting with all that weight will do you no good when you hurt yourself and can't workout for weeks. Also, why hold on to the bar when walking? We don't need one when walking anywhere else. If the speed is too fast just slow it down and walk how you usually do.
Most experience people lose their form as they fatigue but it is important to hold on to the proper form through the exercise. If you need to lower the weight or speed, then do so. Your body will still transform with time but at least you won't have to worry about hurting yourself by doing something you shouldn't have done.