What would the advantages of a local macro be? Are there things you could do with lifecycles or mixins or something that you can't do here that would be useful? I really like the idea of this being at the component level and not having to extend the template spec.
btw, I'm going to investigate if there is a way to support generics or extended prop expressions beyond what var: supports for this. Although, I've been using an implementation of this for months and months and never needed it. You can always make it a component if you really need to, but the simple support using render variables is really nice to have as an option.