Everyone's run into this at some point: The guy who can do 100 pushups or 20 pullups. When you finally see this guy do them, he does them with such poor form that you just roll your eyes. He's sacrificing proper form and control (worst of all) to get the rep count. Every atheletic endevour loves statistical brilliance and doing the most of anything counts for so much. Trouble is, this fixation with getting the most reps out doesn't build strength and it often leads to pain.
I'll be the first to admit, I like having high calisthenic rep counts too. Still, the point of the exercise is to build strength and without proper form, that may not happen. Proper form ensures that the right muscles will get their due stimulation from the exercise. If your ass is in the air or if you're just lifting your chest and head rather than your entire abdomen just for the sake of getting to 100 reps, you're not getting the benefit. You're so focused on that rather than strengthening that the motion is a wasted effort.
Worse yet is the guy who'll sacrifice the control for the speed or ease. I see this a lot on pullups. The person will get their chin to the bar and then let themselves drop rather than control their weight downward. It's bad enough that they're loosing the stimulation on the lats from letting themselves drop but they're also risking INJURING themselves. One point of exercising is to injury-proof yourself, not to induce injury. Remember this because I'm getting tired of repeating it...