On the Subject of Hirelings

NPCs should be available through plot, hiring, Leadership-esque feats and class abilities, and as rewards for adventures.

Because these NPCs are generally rewards for a particular character or are tied to a character's class abilities, they can be run by that player. This avoids having to share credit, XP, and screen time. Just give them a brief bio & simple character sheet and pass that to the player. GM only roleplays them, and may offer suggestions on behavior.

If the NPC is a driver of plot - a deposed prince, a cursed wizard, a questing paladin - that has an adventure in which they're deeply involved, keep them as a GM NPC, but have PCs dictate their actions. Use them sort of like a FF summon or a set of special moves. PCs can call upon their special powers once a scene, or they provide an encounter effect (like a healing aura, e.g.), so they don't steal attention.