A case where the "only 1 instance" assumption can break is on a styleguide page, where you might want to include multiple instances of a singleton element to sh

A case where the "only 1 instance" assumption can break is on a styleguide page, where you might want to include multiple instances of a singleton element to show it in different states, say for your #header for example. Multiple elements w/ the same id will pick up css styles, though.