I can see where this would be a useful feature, but, equally, i'd see why it it should have user control and default to off. I know the problem, and it's especially expensive when roaming. I usually make sure data is off when travelling so that it will only use Wi-Fi, and only when I choose, too.

Apple is being sued for introducing a feature in its mobile operating system that uses mobile data when wi-fi coverage is weak.

Wi-fi Assist first appeared in iOS9 and allows devices to automatically switch between wi-fi and mobile data.

A class-action lawsuit filed in California alleged that some people had run up large mobile data bills as a result of the feature, which was enabled by default. Apple Sued Over Wi-Fi Assist Capability [bbc.co.uk]