Sometimes my phone latches onto a wifi hotspot that I've connected to before, such as on the train, but most public wifi requires a click-through pinky-swear agreement to behave yourself while on their network (which I'm sure would hold up in court, right?) so that means I have to open my browser, wait for the redirect page to load, tick a box, tap a button and wait for another page to load, and *then* I can start using data. When I'm only walking past a cafe or park, this is worse than pointless - it's obstructive.
My point is that my phone is designed to use wifi when possible because it's supposed to be faster, but a lot of the time it ends up being slower due to click-through terms and conditions. My wife knows this. She lives by this observation so much that she never turns on wifi on her phone if she can avoid it. To her, it is unquestionable truth that "wifi is too slow". It shouldn't be, but it is. I have only recently convinced her to turn on wifi when she watches video or updates her software (which she also does only when forced). I live as if wifi is the best option. I leave mobile data turned off unless I absolutely need it. Deb leaves wifi turned off because she has a far more pragmatic view of her phone. Either it does what she needs, right now, as fast as possible, or it's broken. She may have a point.
Mokalus of Borg
PS - The only advantage wifi seems to have is that it's cheaper.
PPS - That cost saving comes with a trade-off for convenience and (sometimes) security, though.