Generally speaking, people don’t use the port on an iPhone for anything other than charging. The fact that it supports USB v-anything is mostly unnecessary cost.
If you are thinking in any way whatsoever about the non-wireless data transfer speeds on your phone, you are not the market or user it was designed for. Same goes for external monitors on an MBA.
I agree that USB isn't commonly used but that's kinda the point - it doesn't hurt apple much to do it (and they're not saving anything remotely significant), but they still do it to force the few who do want to use fast USBs. A more common example where nearly everyone would notice would be the abysmal ram in their phones until very recently.
If you are thinking in any way whatsoever about the non-wireless data transfer speeds on your phone, you are not the market or user it was designed for. Same goes for external monitors on an MBA.