It makes the iPhone useless in busy cities.
Everytime you are playing on 4G, and you walk past a wifi connection you've once used, like a cafe, public hot spot you or a restaurant/ bar, then your internet will cut out and be overriden by Wifi. It would be okay (although pointless) if these connections were secure and connected instantly for the small number of seconds you walked past, however they are all open wifi connections with additional verification. Anything running on 4G will instantly cut out, every minute or so.
So, rather than swiping up and turning off wifi (which is funnily enough, exactly what I want to do when I press the Wifi off button), I have to close my app and go into settings.
What a stupid, poorly thought through concept. There is absolutely no reason at the very least not to add an additional force touch option to fully turn off wifi, or to restrict the auto-connect option to secure wifi connections.