Depending on what the classes in PHB2 actually are, this allows them to release the power sourcebooks on a faster schedule. You don't want to put out an arcane power sourcebook when you're sitting on an arcane class that a lot of gamers will want to play, but won't be out until PHB3 because everyone wants you to squeeze in psionics.
This is more or less what I explained when I predicted just "arcane, divine and primal heroes" in PHB2 in the latter pages of the class speculation thread.
Apparently, four or so classes is enough for them to feel like they've got enough material or covered enough bases to release a power source book. They may not feel like "filling in the grid" with every role for every power source, but I think they do want to have the bulk of a power source's planned classes out before releasing a book that is supposed to be the be-all and end-all for that power source.
This year, we got Fighter, Ranger, Rogue, Warlord and so they can release the Martial Power book. We have warlock and wizard. They'll be adding swordmage. Then early next year, we'll have bard and probably sorcerer if it remains arcane. We have cleric and paladin, and theurge and inquisitor speculation seems likely to me for the PHB2. Then we'll have Druid, Barbarian, Shaman, and probably one other primal class, the 'W' class.
So they'll have 4/5/4/4 in terms of the number of classes in each power source presented so far. That frees them up to release power books for each of those. Arcane, and then probably divine in the summer or fall, and primal in the fall or early next year.
Doing it this way actually provides more options in the short term, if you include the options in power books that they can now release because they've got most of a power source's classes described.
I also think it's better for psionics in general. If you're a psionics player, do you want them to do one or two classes at a time, spreading the classes, feats, items and other details of psionics between two or three PHBs, plus a power source book, that you have to flip through and lug around with you? I personally prefer the consolidation of the core classes and options in as few books as possible for each source.
Since the PHBs clearly aren't going to be an annual thing, anyway, we could very well see PHB3 before Eberron. Maybe it was just my assumption, but I thought we'd be seeing a PHB roughly every June, yet it's coming out one quarter earlier. If PHB3 follows hot on its heels, that means we could be seeing psionics when we expected them, just not in the book we expected.