I'm a little conflicted.
On the one claw, I think "Necromancer" is a pretty distinct archetype from "Wizard." Likewise, "Dark Knight" is a distinct archetype from "Paladin." So those archetypes, I think, are "worthy" of their own class. If also-rans like Runepriests, Seekers, and Ardents can get coverage, certainly they are worthy of the treatment.
On the other claw, they certainly don't need their own classes. A necromancer and a wizard are certainly similar enough to split the difference with fluff, a build option, a few power choices, some feats, and a PP + ED chain. They also gain utility with this model: not only can existing characters dip into these new abilities (through retraining if nothing else), but it's more future-proof. When they support the Wizard, they'll also be supporting the Necromancer, avoiding design cul-de-sacs like a new psionic class released after Psionic Power.
I mean, what, mechanically, should be the difference between the Wizard and the Necromancer or the Paladin and the Blackguard? And why can't that mechanic gap be covered by expanding existing options rather than by trying to re-invent the wheel? If you made a Necromancer class, would it be much different from "A Wizard with a Necromancy school?" If you made a Blackguard class, would it be very distinct from "A Paladin Build"?
As much as those archetypes certainly could warrant their own classes in big colorful letters, I don't think there's a problem with rolling them into existing classes, and I do think you gain a lot of added versatility and future-proofing by doing that. Existing and future characters can still make use of the rules being published.
I'm still not sure it's the best option, but I'm certainly willing to give WotC the chance to show me how it's done on this one. If I can build a character that is a Necromancer who doesn't need to rely on any of the usual Wizard character space, I think that'll satisfy my requirements.