It is also a question of what your rules are meant to represent. The base assumption in the discussion thus far is "adventuring people in a D&D style game" -- why should someone who is just like the PCs operate differently mechanically then said PCs? If they do use different mechanics, then there is an unnecessary breaking of the world into artificial segments.
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One of the reasons I find Dungeon World really jarring is that it doesn't seem to have a different assumption like above but has different resolution mechanics for enemies and PCs. i.e. Mechanically an Orc doesn't really attack a PC (no one rolls for the orc) the PCs can only fail to avoid getting damaged by the Orc.
What happens if the PCs capture the Orc and convince it to fight for them? Damned if I know.
I don't know Dungeon World all that well, but I assume that a captured Orc fighting with the PCs would be similar to a gang in Apocalypse World.
Yeah you don't need to know exactly how Kratos the indomitable got a +10 to attack, but if you say he is a level 5 fighter, you should have an idea of how he could get there.
build rules drive and determine everything that follows. If class, levels, etc. cannot be the same then...
...it only follows that none of this can be the same either. The question isn't whether or not the NPC Thief has an attack bonus, it's whether the NPC's attack bonus is the same as that of my PC Thief assuming all other things (e.g. level, stats, magic) are equal.
"Level" and "class" are purely metagame devices. They are components of a set of rules for building player characters.
In a system that uses a different metagame device to build NPCs, it doesn't make sense to ask what their class or level are, any more than it makes sense to ask what the "class" of an AD&D wyvern is. (The only version of D&D to use "classes" for all GM-controlled personae is 3E.)
If you've got an NPC Thief in a 4e party - let's say lowish level for simplicity - and you turn it over to me as a PC because my original PC died and there's no way to get anything new in, how many things about it will mechanically change? Do I-as-player have to go back and rebuild it from scratch?
if for some reason you need to hand control of an NPC to a character, that character should look like a PC in terms of how they were built. I agree that this is the ideal situation, and this should be something you can do without a lot of hassle.
In AD&D it can easily happen that a being under the GM's control - eg a horse or mule or ogre or gelatinous cube - can move to player control (via capture or taming or a Charm spell or sundry other means). But in AD&D very few beings under the GM's control are created using mechanical build rules that emulate the player character build rules. Even when it comes to a being of a PC-eligible race, they do not need to have a class and level (see eg the Dwarf, Elf, Men etc entries in the Monster Manual) and even if the GM does choose to build them in that fashion s/he is allowed to stipulate ability scores without having regard to the rules that govern PC build (see eg Gygax's DMG p 11: "You should, of course, set the ability scores of
those NPCs you will use as parts of the milieu, particularly those of high level and power. Scores for high level NPC's must be high - how else could these figures have risen so high?").
Here are the rules from Classic Traveller (from Book 1, p 8 and Book 3, p 22, 1977 edition) for when the players have their PCs hire a NPC:
Sometimes (often) players will encounter people not manipulated by an actual player. They may be thugs or assailants. They may be potential hirelings or employers. In any case, their skills and abilities should be determined using the character generation procedure, and noted for the effects they may have on play.
For example, a starship captain may be looking for a crew for his ship, in which case, the referee would generate characters until one occurs with the required skill (such as navigation, medical, etc.). Generally, the first appropriate character to be generated would present himself for employment, and if not accepted or considered suitable, an appropriate delay would occur before another presents himself. As an alternative, the referee might simply generate a character and assign him the required skill, plus perhaps 1 or 2 more.
When travellers require employees, for any purpose, they must find them in the course of their activities. This may require advertising, visiting union hiring halls, or active efforts in barrooms or clubs. Hiring is done by stating a requirement to the referee, who indicates persons presenting themselves for employment. The interview consists of generating the person's characteristics and experience.
These rules contemplate that NPCs may be generated using the PC-build rules, or may just be created by referee stipulation.
RuneQuest is a well-known FRPG which was one of the first to use the same mechanical framework for specifying all beings in the game system, so that they all feed into action resolution in the same way. But it does not require that all beings under the GM's control be generated using the PC build framework.
Personally I don't regard it as a very important desideratum of a RPG that a GM-controlled entity should be easily amenable to slotting into the player-side features of the game such as (eg) balanced character building, clear process for character advancement, etc. As I said the only version of D&D to attempt this is 3E, and it generated a lot of complexity (eg ECL rules) and weaknesses in design (eg undead with too few hit points because of their lack of a CON score).