The reason they decided to make them was innovation for the sake of innovation. Halo all the sudden needed AA's to keep up with the pace of modern gaming and its needs. AA's unbalance the game if they alter base player damage or health, and I hope 343i understands this. Movement and stealth/detection/distraction AA's are fine. I wouldn't say they are 100% balanced. Nothing is truly balanced if both players spawn with different abilities. It's only semi-balanced and made to seem completely fair on the surface because "well yeah you can choose it too." But in reality you do lose engagements purely because of AA's, even when you aim better, strafe better, and generally fight better than the opponent. And you lose because you didn't pick the right one for that moment, thus, not rewarding basic player skills, such as aiming in a shooter, but rather menu selection options which is bizarre. Halo for the most part has lost its satisfying arena feel. It's still fun, but at the same time way more frustrating, and also I find myself owning people and feeling the "my god this is cheap but I love it" rush.
*armor locks*