Form is extremely important. I’d say a 9 on a scale of 1 to 10. First of all, it helps prevent injury. Secondly, you get more out of the workout if you are using correct form.
Let’s say you’re doing bicep curls with a heavy weight and you find yourself using your torso to swing the weights up. You need to use a lighter weight. During a bicep curl, your elbows should stay completely stationary pinned to your ribs. They shouldn’t migrate forward during the exercise. The point of the exercise is to work your biceps – which you’re not really doing if you’re swinging it up.
You either do low reps with heavy weights or high reps with lighter weights (which I think is a waste of time for a healthy, uninjured person and is typically only done by misguided women that “don’t want to get muscular,” but whatever).
Here’s a cardio example: You’re doing jumping jacks – seems simple enough. However, doing them correctly with maximize your caloric burn. Just flailing your arms and legs out won’t burn nearly as many calories as keeping your movements controlled, getting a full range of motion, and keeping all of your muscles (arms, core, and legs) engaged at all times. Your body should be working against itself in order to burn plenty of calories. Additionally, flailing your arms around is a good way to injure your shoulder, especially if your muscles are not properly warmed up yet.
Just going through the motions or trying to reach some number of reps by sacrificing form isn’t smart. If your form is good and you’re using the appropriate weights, you’ll get faster and better results. Period.