And what if you do need it?

This argument made me remember the time when MMORPGs started becoming mainstream in the late '90s/early '00s. I have friends whose ISPs over saturated their connections. Those people end up with 1kbps up/down, which is a big downgrade from their "up to" 256kpbs at that time. And yes, their game is basically a slide show.

Just like your logic, it's not like everyone will be hogging the line, you ask for a website, you wait for it to load then you spend more time reading that site than asking for another website. But then things change and now everyone is sending traffic all the time...

And besides, if you don't care about the bandwidth, USB 3 is compatible with USB 2 so just plug the USB3 into a USB2 port and call it a day...

OP's or Motherboard Manufacturer's logic is sound, motherboard manufacturers decide how to split the PCIE lanes. So the number of USB 2/3 ports are defined by the manufacturer (vs available lanes). If you value USB3 more than having more USB ports, I'm sure there is a mobo model for you. While those people who buy high-end mobos wanting high-end performance gets theirs as well...