Dark Psion said:
... And for the Ninja,... and for the Warlock,...and for the Marshal.
That's three suplemental books right there, why not one more?
I think Merric covered this pretty well, actually: the Ninja doesn't need as much space to sum up his abilities as a Psion would. Warlock and Marshal probably the same as well: by level, you'll get a tighter stat-block out of them than a Psion if you have to explain all the powers, focus rules etc they have running.
The same factor prevents, for example, the Incarnum or Tome Of Magic classes appearing in such a product. There's just too much you'd need to explain before you could successfully use it: any new spell system will always have too much baggage.
(I'm reminded of the RPG stat blocks in the DDM packs, which feature Psionics and other non-core classes, races etc in their makeup: but the abbreviated nature shows the difficulty in doing that with space constraints.)
Dark Psion said:
The problem is that there are those who seem to forget that the Githyanki and Githzerai were created to be the ultimate Psionic adversary.
Well, they have (and I suspect always will have) their spell like abilities listed as "Psionics" in the MM: no-ones forgotten their history per se., but having them be Psionic and having them with classed from the XPH are not inseperable goals. (And if they are, why the hell are Gith, Mind Flayers, etc in the core Monster Manual?)
For what it's worth, in my own campaign the Gith are an extinct race who have recently returned following weird bloodline shenanigans: and in doing so, the first Gith has also been the first Psion in an age. So I'm not here to say "Psionics are rubbish", and I'm not opposed to slipping a Soulknife into my own campaigns. ;-)
Dark Psion said:
But then they also forget the Githyanki worship no gods, so you really can't have a Githyanki Blackguard. Why not use a Psychic Warrior/ Lurk combo for a Captain, it would be much more interesting?
As mentioned, the Blackguard requires no divine patron: heck, in core D&D, gods are by no means obligatory anyway, so a Githyanki cleric wouldn't defy all reason either.
It just comes down to portability: A Blackguard I would feel safe about dropping into pretty much any game I was running, a Warlock I'm pretty sure could bes ummed up in a short block, a Spirit Shaman might take some doing but could maybe be managed. But including your dream XPH/CP class character either requires me to have two non-core books rather than just the one to use the stat block or requires a damn large amount of explanation so I can use it with nothing but the core rules.