Tiburon Silverflame wrote:
I think the multiple PHBs relates to allowing the core to expand in a controlled manner. 4E is doing a major housecleaning...indeed, one could draw the analogy that they're tearing down the house and rebuilding from the ground up. Thus, yes, each PHB (at least for a while) will tend to focus on building "new wings", by defining new sources and classes. The focus, I expect, will be on base classes. Splatbooks probably will exist, focusing perhaps on paragon and epic paths.

I see a couple of advantages to a periodic PHB, as opposed to focused splatbooks.
1. Less lame classes, less fluff. Its probably easier to come up with one or two core, and two or three prestige, classes for each of four different power sources than to try and come up with six core and twelve prestige for a single source.
2. Assumption that most people will have all the previous 4th edition PHBs. This will let them not only add spells to existing class spell lists from all previous PHBs, and include spells from previous PHBs to new classes lists. One of the frustrations with the "Complete *" series was that, with few exceptions, classes with a limited spell list were restricted to spells from that particular splatbook plus core, and other splatbooks can't add spells to that list.