If stun is the only setting available on wide beam mode, then it won't do much good against a frontal assault by armored opponents (that is, Klingons, Jem'Hadar and so forth). And the very few times we have seen wide beam or multibeam modes, they have been with the stun setting - allowing us to think that a technological limitation indeed exists.
What about "The Man Trap"? IIRC, the stun setting just sort of... dazed Professor Crater. (I know, early episode and all... )
Excellent example! I was going to amend mine with "Gambit" where Picard's alien sidearm stuns Riker for just 40-50 seconds, just enough to fool a group of people in a hurry yet still eager to see Riker go down for good, but "Man Trap" beats that one.