It's pre-emptive for now. If there is a requirement for a span of 2/3 of a given container then it will be needed again. I anticipate there are other cases we haven't thought of yet since the medium view fluid support is pretty rudimentary, so the placeholder is there once the other cases get identified. I haven't exhaustively applied support for all 2/3 scenarios as I'm trying to keep the CSS as small as possible and limiting it to what people actually use (until we get a proper fluid grid in v4.0).