Ah, NPCs... In table RPGs, they're the life of the roleplay. Without opposing forces moving in the background countering the movements of the players, without the quest-givers wanting something so much that they're willing to employ the players for it and without the life of beings more than those of the players a world would be so barren. Without those who react to what the players do, without those who pose a threat for the players to deal with and without those who maintain the basic rules of any world, the enjoyment of a roleplay simply wouldn't be there. That goes for any being, the animalistic intelligence of blood-thirsty monsters as well as plain scared civilians fleeing from it. It is always up to the GM to make these as enjoyable as s/he can, for it is eventually they who decide if this is an entertaining world to be in.

In play-by-post, it's an entirely different story.

NPCs feel kind of out-there in such. Most RPs I've been in mostly focuses on player interactions and in those NPCs feel... less like they belong there, in some way. It is like a must for each player to need to adress another in each post so that they get something to react to. I prefer when all NPCs are under the control of a single GM, but that puts so much pressure on the GM in a world where otherwise the games play themselves simply by players continiously posting. In such events, NPCs become breaks which slow the roleplay down and less enjoyment is derived. Another way to do it is to allow players to create their own NPCs and control them, but that becomes a situation where the player plays the NPC much like they're playing a second character, which I discourage. It also becomes an issue where people starts to wonder "How much is OK for me to do?" ... I honestly have no answer to how to handle NPCs in these kinds of matters. They're never as intresting as the player characters, anyway. A strong GM can keep them on leash and conduct a beautiful roleplay waving around his/her NPCs as a masterful symphony, but I've seen far too many wannabe such crack under pressure and see the roleplay go down under.

Play-by-post Roleplays are as most beautiful when they run on their own without much GM-activity. Hence, I really prefer if it is either with only really minor NPC activity controlled by players and allow the GM to simply play a single major NPC at a time, just like s/he was controlling a character of his own. That way, the GM can post mostly major updates to move the story forward when s/he feels like and not be bogged down by having to play numerous minor NPCs at once while s/he simply allows the players to play themselves. Yeah, something like that. ... Though yeah, roleplays controlled by a strong GM capable of handling a world of NPCs are usually the most enjoyable ones, but not all can handle the pressure. That's about that.