Yeah, there aren't many situations where that really washes - it's a reasonable rule to have in mind when designing your API, but it seems to clash with the pri

Yeah, there aren't many situations where that really washes - it's a reasonable rule to have in mind when designing your API, but it seems to clash with the principle of designing "for your clients, not your data". Your clients are often using a browser and don't get to pick the content-type they request. I guess you could contrive to have /resources/xml/apples and resources/json/apples serve the same content, but otherwise it's query strings or interposing a 'choose a representation for your content' page. Naturally, none of this is a problem if your clients are not humans, I suppose.