I was thinking more local where I'm not roaming but going in and out of areas with WiFi coverage. My broadband provider offers hot spot service around town that is good at times and poor at others. Prior to this feature, my phone would stay with the poor WiFi until I turned WiFi off. Later, back on my own WiFi I would find myself using mobile data because I forgot to turn it on again. So, now, I can have a better experience but I do have to be aware of the potential to use more data. You have a good point about roaming.Most unlimited data plans don't apply to roaming.
That's where the problem arises. You think you are using the hotel WiFi when potentially you're not.
I should add too, when I'm consuming media that requires more data like Netflix or You Tube, I'm doing it with my WiFi only iPad connected to a Wifi service. If the service drops off or becomes poor my iPad stops working. My phone is reserved for quick use like looking things up on the go or GPS or email, things that do not require a lot of data generally. Maybe this would be a bigger issue for me if I consumed more of my media with my iPhone versus my iPad that only uses WiFi.