I like answer one and how they still try to imply that essentials and the old format are both supported equally. But the truest sentence:
BBM
So, the essential-like classes get full support, but most of the powers are back-ward compatible?
If this is the case just say so!
There is a new hexblade build.
The hexblade has a x/encounter ability they get at level 1, and an at-will power, and a level 7 encounter power, none of which can be taken by normal warlocks [the level 7 could be, but requires the pact weapon you wouldn't have access to.] So not all of the at-wills and encounters are reverse compatible.
And in terms of "support" ... I don't since how a necromancer supports the mage. If you have a mage built, you have to build a new one to make a necromancer. That's not support, that's a new option. However, something that the character in question can retrain next level, or take at the next level ... that's support. So, they don't include new builds that would require someone to have a PHB to build completely (like the stuff in Martial Power, for example). Or they may actually include all the rules they need (i.e. hit point progression, skills, etc) in the book, with the only stuff from older books being extra options (i.e. more dailies, utilities, feats, races).
If they created, for example, a new implement mastery ... all the powers would be reverse compatible, but no existing wizards would be supported by the implement mastery (although, unlike other class features, they can at least get that with a paragon feat). A new class build for the wizard is as useful for an existing wizard character as it is for an existing ranger character. The most important support is the stuff that can be used by most character builds, not the options that can only be chosen at character creation. The have made the most important stuff compatible with existing characters ... they just haven't made new builds for entirely new characters that require people to purchase older books that are either not still in stores, or if are, are outdated by lots of errata.
So yes, even if they won't say it, you don't need to own a PHB to build these new classes, but if you have a class from PHB, you can use the stuff in this book.
EDIT:
There is a legitimate complaint to be made for some old builds not getting support, namely there are probably not going to be any powers useful for charisma based paladins (aside from utilities since charisma is a secondary stat), for constitution based warlocks (again, aside from utilities since con is a secondary stat), and strength based clerics. Those old dual attack stat classes won't get support in these books, reverse compatible or otherwise.
