Air 2 came with 8.4.1 and I kept it there. It performs very well on 8.4.1 and there is nothing that 9 has to offer (yet) that makes the upgrade a must have (the Air 2 screen size is just too small for truly useful split screen and slide over,.
My Air 2 came with a diffrent flavor of iOS 8 than my iPad 3 was running, and I had to upgrade them both to 9 in order to restore a backup of my old ipad onto my new ipad.
While the air screen is too small for split screen work where you're using both apps equally, slide over is actually pretty handy. A typical use case for me is to peek at my claendar while answering an email. Ad blocking in Safari is also nice.