So your hypothetical home user has a single IP address and runs multiple web servers. And you feel that "Most home routers" should default to supporting that?
Probably not a classic webserver but appliances with a built in webserver for administration and monitoring are becoming more and more common. It's part of the domotica/internet of things wave. Right now these appliances need to use all kinds of workarounds because they are not directly adressable. For example they relay through a server owned by the manufacturer.