We should also think about whether we would want this to be used on the web for the use cases that it addresses. In other words, would we rather have this, or various properties specific to reflections and other similar effects, assuming we were forced into doing one or the other?
I think this is something we implemented because we thought it was a better solution for web developers than alternatives in other engines (which also aren't cross-browser -- I think WebKit-only).
Do we want to invest in pushing this forwards, do we want to accept the alternatives, or do we really just want to hope the entire demand for the feature (reflection-type effects and similar) goes away?
I don't think we've put much effort into bringing this before the working group or trying to get other browsers to implement it -- so it's possible that we could turn it into something that's cross-browser if we put the effort into doing so. (And it might be better than the other solutions to the same problem that are floating around. Or maybe it isn't...)
But I'd like us to be a little more intentional about thinking about how we want the platform to evolve from the perspective of developers and users and not just from an implementor perspective.