Who pays attention to changes in their phone's status bar?
This bit me when I upgraded to iOS 9, luckily I saw it mentioned in an article a few days later so I turned it off before I'd run up my monthly data allotment. Sometimes my Wifi signal is a bit weak in my bedroom so Wifi Assist would be handy if not for the fact that I have limited data since I'm on wifi 99% of the time and browsing Facebook seems to eat data at a fantastic rate!
What I don't get is how this was not noticed during the public beta period for iOS 9. While I imagine Apple employees involved in testing would have unlimited data (or Apple pays for it and doesn't care how much they use, which is the same thing) that wouldn't be true of some of the developers and general public testing it. Did they tell Apple and were ignored or did no one notice until it went live?
I get that when they add new features they might be unused if it is left up to the users to turn them on, but they should have known this would end up costing some people money and thus the default should have been 'off'. Even those with unlimited data might not want this, if it meant hitting the point where their carrier starts to throttle their data.