Because they're the Swiss Army knives of the future. You can use them to heat rocks, cut through an engineering bulkhead, stun a gangster and, yes, vaporize something -- such as the top of a mountain concealing the entrance to the Talosians lair. I think we seldom (ever?) see Starfleet personnel in their right minds using them to kill. Offhand, I can only think of the salt vampire being killed (in defense of the captain), and even that wasn't vaporization, and it was after attempts to stun.