So when the pet battles system was announced at Blizzcon I was one of the people that didn't particularly care for it, but didn't object to it being in the game at all. I am still thinking of it in the same light, and I am also not one of the people spazzing out because they are developing the Pet Battles system and not something like player housing.
With all that I was browsing MMO-Champion and it was the first time that I looked and saw everything that was going into the system itself. They are really going all out on it. Also if there was ever any doubt that this is directly related to Pokemon that certainly has been shut down after reading the write up. In addition to all of this and I congratulate them for adding something large to WoW. I am thinking it might actually be too much. When you create a Mini Game within a game you always run the risk of it becoming more people than the actual game itself.
Some games actually have come close to having this as an issue. Where people stopped playing the game and concentrated on the game within the game. I am not saying that everyone is going to stop playing WoW and just partake in pet battles, but I am thinking that some of the population will.
Maybe Blizzard doesn't mind this happening, but it could be that argument that the only thing which will kill WoW is WoW itself. Not by releasing lackluster content, not by horrible balancing, but because they introduced a system that too many players decided they want to play around with instead of the game itself.