Designing classes with a rock-paper-scissors philosophy keeps a certain, but necessary imbalance to how classes match-up with each other. While Hunters, Shadow Priests, and Warlocks may be strong classes, they do not have everything. They have absences which other classes can exploit, given the opportunity.
To say that the game is built around group PvP acknowledges the fact that each 1v1 match will be distinct and not necessarily fair. Any one class may have particular counters against another specific class, which puts them at a natural advantage. In group PvP, this doesn't factor in as much because the other side may have the same or different classes and team play and tactics have more bearing.
Balancing classes in a 1 to 1 setting was something that was happening with the Shaman and Paladins. This conflicted with how all of the other classes were being balanced in a group setting, as changes could be made to a class with the overall game balance in mind, not one particular class such as was occurring.
Yes, it is much more abstract than a 1 to 1 setting, but it also yields more flavor and function for the individual class and for group dynamics in general.