I'll agree with the general sentiment here, with a caveat. I think we are moving a bit towards the latter 2E "rules bloat."
Yes, the extra rules are all optional (as are the core rules, but there is "optional" and there is "optional"). However, more and more WotC books are adding in parts of those optional rules into books that aren't tied to that rule set.
It's obvious that Complete Psionic will refer to the Expanded Psionics Handbook. Maybe even the Complete Mage should reference the Complete Arcane (although I think the marketing didn't sell it as such). However, it's happening elsewhere. Eyes of the Lich Queen uses the Tome of Battle (incorrectly I might add). A "pseudo core book," Magic Item Compendium, references a gazillion rules sets (incarnum, psionics, Tome of Magic, Eberron).
While it doesn't make those rules less optional, it makes them feel less optional, If I pick up the MIC and see a reference to incarnum, I want to know what its talking about. Why are there no rules covering it in the book? When this happens it seems that a rules is "core" and I'm missing it somehow.