In all seriousness, I've never understood people's issues over taking prisoners back to an authority. It's an imaginary game where such things can be hand-waived as, "We take him back and hand him over to the law, and then return to the dungeon," and then continue the adventure. Ooh, five seconds, so much hassle and bother.
Nod, in my campaigns, prisoners are typically handed over to the proper authorities. Their eventual fate varies from being pardoned (happened for some bandits who were coerced) to being hanged to waiting in a dungeon for a prisoner exchange.
My campaign centers around a war with neighboring country, so some opponents are "irregular" forces of the enemy.
With humanoids, on occassion peace treaties have been hammered out (between NPC leaders with the PC's naturally as go-betweens) and lasted for some time, with prisoner exchanges often part of the deal -- in one case, a PC was rescued from the Goblin King in this way.
The significant cases I recall where prisoners were not handed over to the authorities:
-- Meepo was "captured" in the Sunless Citadel and joined the party. When more kobold prisoners of the goblins were rescued, they also joined the party, until the goblins were overcome -- and Meepo became Chieftain of the Kobolds, making a peace deal with the hoo-mans of the surface world.
-- A hobgoblin officer who the PC's fought several time was on the verge of defeat and sued for peace. He asked to be able to depart, with his wounded, females, and young ones. The PC's agreed. Later on, they met him again, in a different dungeon, and said "no quarter", as he well knew would be the case.
-- The PC's captured some orc soldiers at the Mountain Door in the Forge of Fury. In my campaign, these orcs were actually N alignment, worshippers of the Cave Goddess. They asked for a prisoner exchange, and the negotiation eventually turned into a broad deal.
The PC's gave the orcs:
(1) their orcish prisoners (mostly still unconscious the morning after)
(2) two barrels of good stout ale they had on a mule
(3) general agreement to discontinue hostilities
The orcs have the PC's:
(1) their prisoners -- two gnomish prospectors the PC's didn't know were missing
(2) guidance through the orc's lair to the deeper dungeon behind
(3) an outline of all the traps and other monsters deeper in the dungeon that the orcs knew about
(4) general agreement to discontinue hostilities