Not sure your example fits completely, because Finn is also a traitor. Of course, it’s unlikely (and possibly silly) that someone would get into a “fair fight” like that stormtrooper does (luckily he’s the only insane one of his entire squad 😄 )
I don’t think his weapon was necessarily designed to fight the Jedi: just for close quarter combats, possibly as a kind of shield too (if it can deflect lightsaber blades, why not plain laser beams?)
Yeah, I guess Abrams thought it would’ve turned out as a “cool scene”, but at least it kind of works as a way to develop the character, making Finn face his former “comrades” while giving him a chance to learn how to handle a lightsaber.