This change is being made to further emphasize the desire to kill bosses for the items they drop. In the 4.3 raid, tier sets can only be earned from boss drops, and as Raid Finder will allow for just about anyone to get a chance to kill bosses, we think there will be less need for Valor overall. We want to try to match the lower desire for VP with a slightly slower acquisition rate.
We appreciate your thoughts and constructive discussion on how these changes may affect you and your guild once 4.3 is released.
We're also making changes to Conquest, recently announced, found here: http://us.battle.net/wow/en/forum/topic/3595670888