It's a well-worn cliché of film and TV that, when facing off against a main character, villainous goons will use whatever strategy and weaponry makes for the most visually impressive fight scene that the hero could conceivably win, instead of simply all repeatedly shooting the hero in the chest (feel free to add a TV Tropes link in if this has a name, I'm sure it must).
But is there any explanation within the logic of the film for the scene outside Maz's Cantina, where a stormtrooper squares off against Finn (who I believe was already nervously wielding a lightsaber), calls him a traitor, and switches his blaster to electrified/melee/crowd-control mode - fighting him hand-to-hand in a duel Finn could win where this stormtrooper was the only combatant without a lethal hand-to-hand weapon - instead of simply shooting him?
I don't remember seeing any clue that the stormtroopers had planned to take Finn alive. The other stormtroopers didn't seem to be holding back from shooting at him, with characteristic accuracy.
I seem to remember seeing something (I think it was a "riot control stormtrooper" toy?) that implied that this stormtrooper might have been carrying only riot control equipment, that couldn't be used as a blaster - but that leaves us with the questions of a) what someone equipped for riot control was doing on a battlefield, and b) why they chose to challenge the one person on that battlefield who could fight back in a hand-to-hand duel. It really looked like a blaster being switched into an alternate mode to me, which would explain question a) - but not b).