The purpose of the entire exercise in humiliating the Enterprise officers in front of McCoy was to break his resistance to serving The Platonians willingly, as a skilled physician. While they were fully capable of forcing other life forms to act as puppets, this doesn't include tapping the skills of the subject as part of the puppetry.
If it had been in line with their goals, they could likely have destroyed The Enterprise or otherwise rid themselves of the rest of the crew as you suggest, but this would remove their main leverage in influencing McCoy.
This is corroborated by the episode's dialog:
KIRK: No. You're the doctor. They don't want to force you. They need your goodwill. They're trying
PARMEN: Captain, go while you still can.
KIRK: We're not leaving until McCoy is released.
PARMEN: This is not the Enterprise. You are not in command, captain.
PHILANA: Why even discuss it? Get rid of them.
PARMEN: No, my dear. That might offend the good doctor. You wish to stay? By all means. You can help us celebrate our anniversary. In the process, I hope we can persuade you to join our tiny republic.
MCCOY: You won't persuade me.
PARMEN: I think we will.
Humiliating Kirk and Spock gave The Platonians a venue for demonstrating their strength and indulging their vanity. In their arrogance, they believed that a demonstration of their strength was the most direct means of demonstrating the futility of McCoy's resistance, and gaining the services they desired.