Falling from standing height is really dangerous, especially for people of full adult height (acceleration due to gravity is worse the higher your head is, and your head's the main concern—it's part of why toddlers can fall over all day, even on somewhat hard surfaces, and unless they hit just wrong they'll be OK). "Stun" that immediately dropped people would still kill and cripple sometimes.

[EDIT] especially in Star Trek where the stun sometimes tosses people off their feet, too, quite a few of those people/aliens would end up dead or requiring a visit to the med bay, IRL.