Sometimes, sure. If all you want is a monster that will be played for a few rounds.
But sometimes you want that with PCs too. If you're teaching a beginner, or playing a one-shot, or you're just a rules-lite kind of person, you probably don't want to pick a full sheet of different abilities, feats, backgrounds, etc. You want to make a couple of quick choices and play.
I don't consider the distinction to be arbitrary. PCs and NPCs exist for ENTIRELY different purposes. They don't EVER fulfill the same role in the game, thus there is IMHO no reason at all that they should follow the same rules from from first principles (in terms of how they work IN PLAY there are of course good reasons for their rules to be similar, though I would point out that games like Dungeon World take the concept to the extreme, DW monsters are NOTHING like PCs at all, not even in terms of basic mechanics).
Of course it is possible one can desire to have a simple PC. I would think that a simple class is appropriate. Clearly the 4e devs thought that was a good idea when the designed the Essentials Fighter classes (Slayer and Knight). I don't see how this has much bearing on monsters/NPCs though, because again, the purpose of an NPC is so fundamentally different from that of a PC that the mere happenstance of a simple PC class and a simple monster having similar complexity is not really relevant any more than a simple spell having complexity similar to a thief ability is relevant to anything.
Conversely, sometimes as a DM you want a major NPC, DMPC, BBEG, or other well-developed stat block. And sometimes PCs want to play unusual things.
OK, but again, what compels one to consider a PC stat block to be appropriate for an NPC, well-developed or not? What would make one believe that a 'class' intended to model a monstrous NPC be useful as a PC class?
The point is that the amount of time one wants to spend creating a character is not solely or even primarily a function of whether or not that character is a PC or an NPC/monster. It has far more to do with what kind of game you're running and how much time you have and what you want out of that character. So why should the rules enforce an arbitrary distinction between PC and NPC? What is gained by doing that? Shouldn't they simply be designed so that you can create a basic character sketch quickly, and add detail if you want it, regardless of who you are?
First of all I don't agree with your initial assertion here. DMs create MANY MANY more NPCs/Monsters than players create PCs over the course of play. I started a campaign last night and the PCs have already interacted with at least 20 NPCs and killed 8 monsters in one evening of play. This is typical. I will generally at least make note of a dozen or more NPCs each session and I may well provide stat blocks for ALL of them, though probably half is typical. Thus time is a major driving constraint when I build NPCs. OTOH I usually spend at least an hour making a new PC, sometimes more.
The distinction between a PC and an NPC is in no way shape or form arbitrary. In fact it is one of the most important distinctions, perhaps THE most important distinction, in all of RPing. It hard to even answer what is gained by using different rules for them. It is like asking what's to be gained by having different rules for the offense and the defense in American Football. The rules virtually MUST be different as the things themselves are of a wholly different nature.
NPCs are create, exist, and are often destroyed or removed from play quickly and regularly. They don't need to have mechanics that allow them to deal with any arbitrary situation, manage resources over an adventuring day, cooperate with other PCs in a team while filling a unique niche, etc. They in fact need to be able to present a credible and immediate threat, be easy to generate, easy to run, easy to customize and reflavor in arbitrary ways to meet the DM's needs, etc. These two sets of requirements are clearly very different and any assertion that an equivalence should be drawn should be approached with great skepticism. As a systems engineer I'd say that such a choice is a hasty generalization.
Now, all of this doesn't PRECLUDE commonalities of PCs and NPCs from being exploited. It just means that the similarities should be factored out into a common area of the rules. 4e for example does that quite well, almost all 'in play' rules simply deal with 'creatures' and 'characters' and 'friends' and 'enemies'. NPCs use the combat system exactly like PCs with a few very specific minor exceptions. They have attributes which work like PC attributes, and even skills and powers like PC skills and powers. Many other mechanical details are shared. However their generation systems ARE quite different, as befits their different purposes. In point of fact it is QUITE possible to make 4e PC-style NPCs if you WANT to do so, and you can even find in DMG1 a set of rules for making 'monster' type NPCs that use basic class mechanics from PC classes in a limited way (using class templates). This is further developed in DMG2's CC rules. Still, these are all fundamentally still stat blocks, not class based creatures.
Just to specifically address the point of "well rounded NPCs", those with a high degree of development. Again, I don't see that PC rules are particularly beneficial for them. Even these sorts of NPCs are created for a purpose. They may be recurring significant characters in the story, but its not necessary for the rules to govern their use and acquisition of powers and resources. The DM should be free to structure these villains and allies as he sees fit in order to play their parts effectively. In any case PC rules don't really help you do that very well. They are mainly focused on regulating the rate of acquisition of new capabilities, something of little use to NPCs. These rules also rarely provide much insight into the actual resources of a given character. For instance a 4e 'wizard' NPC might have a wide variety of ritual magic, unusual items, access to resources and allies a PC wouldn't have or need, etc. Many of these things in fact need not be specifically nailed down and defined. Who knows what sorts of odd powers a mad demonoligist might have for instance? No PC is going there! If threatened who knows what sorts of resources this NPC might muster up using his forbidden talents? Certainly to assume that the DM is going to think of every option, let alone that a class designed to limit the power of a player's character is going to have the appropriate mechanics for this seems unlikely. There is of course no reason not to be guided in what said NPC can do by examples of what PCs can do, but this should really be a mere starting point or reference.
