Simply put, pet nerfs are for lazy people. There is absolutely nothing worse than spending hours, or days, or sometimes even weeks to farm a pet, only to find out that it was time wasted due to a nerf. Best example I can think of is the Fluxfire Feline. Sure, it was OP, but it was a WILD pet. That means that every single person in the game had the ability to get it. So where is the problem? Oh, that's right, some people don't want to farm, and so they cry foul to Blizz. "Dear Blizzard, this guy has put way more time and effort into pets than I have, and its not fair that he beats me. Can you fix it?" Sure enough, Blizz implements a nerf, which means that those of us who DID farm, basically wasted their time, and the very ability we farmed for was taken away.
I've been doing pet battles since the very beginning, and in all that time, I've only seen ONE pet that I actually agreed with the nerf, and that one was Murkalot. Every other pet, regardless how OP (Kovok, Direhorns or Flayer anyone?), was beatable if you put the time and effort into figuring out how.