Why? If a user interacts with a site differently on mobile than on their desktop machine then two separate interfaces is good? That's not just good "UX", it's g

Why? If a user interacts with a site differently on mobile than on their desktop machine then two separate interfaces is good? That's not just good "UX", it's good logic. The main argument I see against separate "views" is that it's an expensive exercise to effectively build two frontends for a single site. However, as someone who has built both large-scale responsive and mobile-specific sites neither is really cheaper.