The change from "aggressive" to "assist" in 4.2 (patch notes here -- http://us.battle.net/wow/en/blog/2993743#classes ) makes hunter pets less deadly to the hunter in PvE situations, and more useful in PvP situations.
I will miss the ability to stand back and have my pet pull every mob in the Barrens, but not that much.
As a longtime reader, sometime poster I have to say this is my favorite Blizzard post ever. Back when I was on my Paladin I remember the first time I ever heard the term 'huntard'. It was used because a pet had caused a raid wipe by aggroing a small army in Upper Blackrock Spire. This was not the last time it happened.
Granted, pet AI has come a long way, but I ask myself "Self, why would anyone in their right mind subject themselves to a stance that when you turn it on runs the risk of your pet running all the way to Canada?" Really. No one is perfect, it's pretty easy to lose track of where they go. If it weren't for pet despawning I'd worry that every time I used my hearthstone that Disney would get ideas for the movie Homeward Bound 3: Lost in Outland. The tagline for the film would go something like this: "The Burning Legion has returned again (because some Hunter's pet aggroed them all through the Dark Portal)."
So what's the main reason that people argue about for keeping aggressive? To find stealthed players. I'm very sorry that you can't detect stealthed players. I'm also sorry that there isn't an engineering plan for x-ray goggles or some sort of dust cloud that reveals the shape of invisible forms, or even better some manner of shot which sends a bright burning light into the air which then remains on the ground and reveals stealthed players in an area around it. Having an unfair tactical advantage is in PvP nice, (and one that requires no effort whatsoever) but especially as Hunters we should recognize that if we're going to complain about unfairness in PvP, that the street goes both ways.
The fact of the matter is that aggressive stance provides very little quality of life improvements that good pet management can't do better. This is why they got rid of it and replaced it with something that might actually be useful. It may be fun to train all the mobs in the Barrens, but I'm not going to miss it either. We've progressed beyond that point.
And did no one else notice the most important part of that blue post? At long last we have confirmation that someone currently at blizzard actually has (and may possibly still play) a Hunter.
I play most every class, and my hunter is among my favorites. I feel like he requires a certain level of communicative finesse in PvP, but I like that. In arena battles, for example, I have 2 or 4 teammates that I'm working with. It's not just me vs. an enemy rogue.
There are a lot of Blizzard employees that refer to their hunter as their main, by the way.