Classes that are better off as NPC classes?

[Nanashi]

Are there any classes that are so singularly focused or outdone by other classes you wouldn't play them, but do think are usable as unofficial NPC classes?

Warmage has fluff that should make them reasonably common spell casters. Their limited list frees up the trouble of having to assign one for them. Could stick some metamagic tricks+reducers onto them and some weird advanced learning to still make them have variety

Marshall, likewise, has extremely simple fluff that just makes them military officers. Mechanically they're easy to run (grant bonuses to nearby enemies, grant enemies a free move action, attack) and can spice up some warrior mobs without introducing a bard and all the casting that implies. (Any easy to run feats that provide party wide buffs to give them besides Draconic Aura?)

Pathfinder:
Skald really only works if you have a melee centric party that can't already rage and isn't casting dependent. That's extremely unlikely for players, but trivial to setup for some enemy encounters. Much harder to prep than the other two since it's a spontaneous caster that needs a spells known worked up though.

[Skyrock]

Healer. All they can do is heal and remove bad statuses, and they don't even get the best healing spells. Just make them truly WIS- or CHA-SAD, and change them to fixed list casters like Beguilers/Warmages/Dread Necromancers so that they don't have to present a list of prepared spells, and they are good to go as NPC classes.

[Stratovarius]

CW Samurai is already an NPC class.

[zook1shoe]

PF has a number of archetypes like that

[Power]

PF Skald is not that bad. Spell Kenning gives him absurd spellcasting versatility and he can use feats to obtain an animal companion or just summon monsters as a Bard. Inspired Rage offsets the weaknesses of summoning as a 6th level caster well enough.

In PF I'd say Monk, Rogue, Cavalier, Swashbuckler, and Gunslinger are like that. There are also a number of archetypes that are so bad only NPCs should be taking them, like Groom on Ranger, Vow of Poverty on Monks, Cloistered Cleric, and probably loads more.

[Nanashi]

Oh, I agree Skald is excellent. It's just so unlikely you'll have a party that can use it to remotely near max efficiency compared to a bard that can benefit just about everyone except maybe straight arcanist (who still loves the saving throw bonus and better to hit on touch spells) an NPC has an advantage there.

Those PF classes are pretty bad, but most of them don't really have anything that makes them really interesting for NPCs over PCs. Maybe Gunslingers if you abuse the personal firearm to let them hit touch AC while only providing ~22 gp of loot. PF Cloistered Cleric likely has NPC members, but not the kind you'd bother stating beyond "can provide spells of up to X level, has Y domains". The archetypes that are actually made for NPCs are all aimed at cohorts.

[zook1shoe]

there's not really any vanilla PF base class that are better off as NPCs, it's the archetypes that can really change that.

there's a difference between 'bad' and 'meant for npc'

Knights of the Inner Sea has 4 archetypes that are for squires.
how consistently do people go out of their way to be a Gunner Squire who helps reload his Lord's guns?

there's some intrigue-based archetypes that are better suited for npcs than pcs too

[Keldar]

PF Swashbuckler amazes me, it seems to be 3 levels long, just like the 3.5 one.  If Swashbuckler Training counted as Weapon Training, it'd be well worth 5 levels though.

[Power]

It's "Swashbuckler Weapon Training" so it counts as a "weapon training" class feature, yes. Still not very good though, since you're playing melee without pounce. You could make a whip-based Swashbuckler with Lunge and Slashing Grace though. The Mysterious Avenger archetype specializes in whips (so you should probably just use an Agile property instead of investing feats into Slashing Grace) but the rename would imply its version of Swashbuckler Weapon Training no longer counts as weapon training (if your GM is not a stickler on this point it shouldn't be a problem though).

I'd easily take the Fighter's bucket of combat feats over this. Or the Brawler.