While strictly true and you could drag up some pretty bad abuses of USB A especially as used for mobile charging, the peripherals / protocols usually just work. But here, if you have a cable in your hand with a USB-C and a DisplayPort connector it's pretty much anyone's guess whether a) it'll work with the given machine at all b) if it does, which version of DP will be supported with the given machine.
I'm confused the scenario you're imagining in which someone both doesn't have their own, known good cable, and also is in a place that doesn't provide a cable, where displayport would even come into play.
I can think of exactly 0. If the projector needs a displayport capable usb-c, it's going to already be attached to the projector. If I need one for home, I'm going to buy one that supports it.
If there are 8 different cable types I'd rather have 8 different connectors, yes. Remember going around the office asking to borrow a cable to charge your 'phone and having to figure out micro vs mini vs sony's weird thing vs apple's weird thing? It was a bit silly but at least having physically different connectors meant that you could tell whether a given cable would charge your phone or not. Now it sounds like that's no longer true.