Another controversial thing about Fourth Edition D&D is that they are planning on multiple versions of the core books over time. Not that they didn't have them in Third Edition, but the kicker this time is several classic monsters and classes are being held over for later books. For example, if you really want to play a monk you will have to buy Player's Handbook II (or III) at some point down the road.
This concept has really pissed off a couple of my friends. They are very indignant that Wizards of the Coast is holding back "the good stuff" in order to extort more money out of them. I am very sympathetic to this point of view. In fact, I shared it when I first heard about this concept.
The more I think about it though, the more I think this new way is more honest. After all, books like Complete Warrior were additional Player's Handbooks in everything but name. New base classes, prestige classes, equipment, etc. Everything you would expect to see in a PHB. If you tended to pick up all these books anyway, this probably won't be much of a change for you
It will add a burden to the casual player. At least a couple of players in my gaming group were the type that would purchase the Player's Handbook and nothing else. They might occasionally use classes or feats from the non-core books, but would rarely buy them. While it is certainly true they could do the same with multiple core books, something about naming a book "Player's Handbook II" instead of "Complete Adventurer" seems to make it more of a must buy.
In the end though, a lot of the motivation for multiple core books does come down to the economic realities of the gaming industry today. Back in the days of First Edition AD&D, selling adventure modules was how the game made money. The rules were the rules and the concept of supplemental rule books was a foreign one. Then came the original Unearthed Arcana. While it was really just a collection of rule supplements previously published in Dragon Magazine, it added elements like new classes to the "official" AD&D rules for the first time. Not surprisingly it sold like hot cakes! It was quickly followed by the Dungeoneer's Survival Guide, introducing the concept of skills for the first time.
By the time Second Edition AD&D came around, selling rules supplements rather than adventuring modules became common place. The reason for this shift seems obvious. If you have a group with five players and one DM, only the DM is buying adventure modules. On the other hand, everyone is buying the rules supplements. Still, no rules supplements sold as well as the core rules. Which is why multiple versions of the core rules makes financial sense for Wizard's of the Coast.
I suppose sometimes I wish I could turn back the clock. Lord knows my wallet wishes I could. Nevertheless, I don't think the idea of multiple core books is a bad one, at least if they manage to keep the quality level high enough.
I reserve the right to start griping again when they try to sell me Player Handbook VII though.