I'm A Banana
Potassium-Rich
pemerton said:KM, I don't want to kickstart the whole simulation/gamism etc thing on this thread and derail it, so feel free to disregard this: but there is a coherent (non-hardcore simuationist) approach which says "Just because NPCs and monsters can do it, doesn't mean PCs can."
There is an issue with that approach, though. You can wind up with things like 2e's drow and their sunlight-disintegrating weapons, things specifically designed, from a metagame standpoint, to only be used by NPC's that create a problem in the hands of PC's, and so need an in-game justification for why they usually wont' work in the hands of PC's. 2e was actually rife with this kind of stuff, IIRC. And it blew verisimilitude out of the water for me.
If the NPC's and monsters can do it, it needs a cause, and that cause should be something that the PC's can interact with themselves.
Now, I don't believe that the rules need to be the absolute same in all instances. Mook rules and elite and solo creatures are all well and good, but the PC's should be able to take advantage of these rules as well (hirelings and combo attacks and powerful rituals/summons), or else I run into a problem of how it exists in one form when the DM is using it and in a completely different form when the players are.
So in the same way that we don't allow PCs to be wealthy monarchs at 1st level (although there may be such NPCs in the gameworld), we don't allow them to be paper tigers (because, like a lot of ECL PCs built using the Savage-Species approach, they won't play very well).
Well, to be fair, 1st level PC's can be wealthy monarchs, at least in 3e. And, in fact, a wealthy monarch should be a valid starting course for a PC, I believe. And I'm not sure they won't play very well, unless by 'wealthy' you mean 'liquid gp resources equal to a much higher level character.' But usually, the wealthy monarch archetype is less about the powerful items they have and more about the role-playing, social control they have....
....but that's kind of a tangent.

Though ECL PC's didn't play very well, this doesn't rule out other ways of making monster PC's play well, and I'm sure they're out there.

I get the strong impression this doesn't really satisfy you, but I can see why the game only sets out to support a certain range of PCs. Not every conceivable possibility has to exist as a PC build.
The issue of monsters as allied that you have raised from time to time, however (and their treasure/equipment presumably feeds into that) is a good point that I agree with.
Well, for me, it's an issue of "why can't I say 'yes' to this?" I'm an actor, my D&D sessions are quite heavy with improv, and one of the rules for improv is always "Don't Contradict The Others." I find this feeds into an immensely satisfying D&D game for me, because it makes me be creative in response to my players. 3e gives me strong enough base assumptions about the bits that make up D&D that I can feel fairly confident in a creature's 'character.' That's me playing it's role, improvising dialogue and world elements as I go.
So if a player comes to me with a valid, entertaining idea, for, say, a beholder as a PC, I want D&D to be able to tell me how I can say 'yes' to that. I don't want D&D to tell me that I have to say 'no' because the beholder will overpower everyone. That's not useful for me. I'd resist D&D telling me that I can say 'yes,' but only with limits (the beholder has to basically be so weak that it's not fun to play, or some sort of genetic mutant who doesn't actually have beholder abilities). That's not REALLY useful for me because it's not a satisfying play experience for them.
The thing is, everything a villan has is an item in the world that the PC's can and should be able to interact with, and everything a villan encompasses is something that the players should be able to harnass as well. If I have to suddenly alter rules on the fly because I can't allow a player to have an ability or be a villan or have a little pet monster, if I have to keep saying 'no' to them, and the reason is transparently one of balance, it becomes deeply unsatisfying. The players know the deck is stacked against them and all they can do is play along with whatever I give them to play along with? That's pretty unsatisfying for my groups. It invalidates the atmosphere of improv that permeates my games now, that allow me to say 'if you can imagine it, I can find a way for you to do it.'
The ability to say yes is something that 3e gave me, and while it didn't always fulfill it's promises (ECL, CR), it did give me a guideline I could depart from. I'm not eager to start saying 'no' to my players any time soon.