MIME types and HTTP content negotiation are good ideas in practice that have failed to take hold on the Web. Arguing that this fact contravenes stuff written in specs from last decade or from findings by some ivory tower group of folks from the W3C seems like religous dogmatism and not fodder for decent technical debate.
That said, I don't think MIME types should be retired. However I do think some Web/REST advocates need to look around and realize what's happening on the Web instead of arguing from an "ideal" or "theoretical" perspective.
Sometimes, I feel like web specs are like battle plans - they don't survive contact with content providers any better than battle plans survive contact with the enemy.