Apparently it was very difficult for GM to get the transitions between cylinder modes to occur smoothly, especially with a manual transmission involved, so I wouldn't call it an easy way to get a benefit. I imagine there's also the added weight, space, and complexity factors. Most automakers seem to be content simply abandoning NA and dropping cylinders instead.
Also keep in mind that in much of Europe, fees are assessed by engine displacement, not MPG directly or the presence of features like that, so that market responds more to getting power from less displacement rather than making higher displacement engines more efficient. In fact I believe the displacement tax is an ANNUAL charge like our registration fees, not like our gas guzzler tax that the first owner pays once at delivery.