While we recognize that NPCs can play an important role in filling out scenes or providing plot fodder, we have implemented a few limitations on their use, to avoid any issues of power-gaming or overshadowing actual PCs.
Players are welcome to use NPCs as scene fodder. However, these should never be FCs or major characters. They are generally unremarkable and dispensable extras who play a minor role or fill in the background. They should never take the focus from the other PCs or have too much impact on a scene.
The one exception to this is in the instance of running a low-level 'throw-away' antagonist for a scene (such as a mugger, for example). Players may use such a device in order to give themselves something to fight.
Ongoing NPC bits may be granted to established and experienced players. These NPCs have wiki pages and may even have separate logins. They interact more on the level of a full character and may have a story and develop limited relationships. However, they are created to serve a purpose or fulfill a plot, and will never take the spotlight from a full PC.