Importance of Proper Technique

Question:

How important is exercise technique in strength training? This may seem like a silly question, but I see a number of competitive bodybuilders at the gym, whose workouts are obviously effective, and they are ignoring all of the technique rules I have read about. For example: no full range of movement, using momentum, swinging posture, high reps, not going to failure, not slowing the movement in the extension phase. Since these workouts seem to be effective, I am starting to wonder if I'm being excessively strict about exercise form with myself and my clients. Can you clarify this matter please?

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