It's no different from the PHb1 classes being exclusive to PHb1, or the PHb2 classes being exclusive to PHb2.
The big question is how much future support will the Revenant and Assassin get? The Gnoll (the only current Dragon race likely to remain in Dragon, and even then some speculate it'll be the final PHb3 race) has gotten no support outside of the original Playing article. Neither have the other Dragon races, but then 2 are definitely coming out in PHb/PGs, and one (Shadar-kai) most likely will, so they'll get support after that.
Will the Revenant & Assassin get support in books, just in Dragon, or nothing at all once the articles are out - that's the big question. Hopefully they'll at least get support in future Dragon issues when stuff like Power book X or PHb X comes out.