That's creator-centric, not user-centric. Art, not Design. This is in fact my main contention with the Dutch School: it's centered on expression, inescapably at the expense of service. Individualism, not Collectivism. Making marks "by hand" because that's the human way. Except the human way of seeing is detached from that, making the human way of marking at best an arbitrary design constraint, at worst misleading.
Anyway, more firmly on-topic: Especially in small running text readers need a much stronger visual clue to reliably discern emphasis than a construction principle that often results in minimal actual difference. Slant is bullet-proof there. Upright Italic sacrifices slant essentially to push dogma about personal expression. This is anti-Design.
One reason to have a separate price for interpolated styles is that clients change their minds about how many they want (or can ask for more later) and they're less suspicious if they've already been told how you like to price things.