Rendering slots and transclusion are closely related, and even conflated at times in the code. There are good reasons for that, but some clear separations of responsibility are in order.

The way it actually works most of the time is that you are interacting with a javascript app that is controlling a "slot". That app can fetch different views for the card in the slot, and generally control the view any way it wants without bothering the context it appears in. Now with even layout relying on transclusion processing, anyplace a card is transcluded, a slot is being created at the point of transclusion.

All this is great, but first, not all renderers need any of this. The XML renderer for example. And the second thing is that when they do, we want to give the card designer the ability to customize the controls that appear within the cardslot. I think this is a key concern when you get to designing the new view and controller actions, in fact I think this idea more or less defines that issue.