I'd generally agree that someone on the ground in handcuffs does not need or deserve to be subjected to a Taser hit. There are exceptions, of course.
I've seen & handled guys in handcuffs who just won't quit. They try to kick, spit, bite, claw and anything else they can. Before stun devices were available, the SOP was just pin him to the ground with a knee in the back and let him exhaust himself. If he was too unruly for that option, you'd resort to "benediction by Mace®" and let that cool him off.
If the suspect is kicking and flailing about hard enough, even in handcuffs, where officer's legs can be injured then tasing him might be appropriate.