They also took out weapon switching. And they more or less admit it is to prevent players from gaming the system. In Diablo II, if you chose to pass on a skill or underpower it, it didn't matter if you could switch to it quickly or not. Builds were set in stone. In Diablo III, everyone at the same level has the same skills accessible. You can change to any other build at any time and be comparably powerful to anyone else with the same gear. While, less important for PvE, it would break PvP utterly if it was possible to instantly switch builds. Imagine a wizard that can throw out all of his alpha skills and then insta-switch to a defensive setup. Or one that can turn on Glass Cannon just for the second his meteor hits and then turn it back off.
Yeah, it would be a mess.
I'm not sure that weapon switch is so game breaking, but the folks at Blizzard are.