[Slots] Why is it only possible to add instances and not just arbitrary frames?

Hi, 

one of the biggest advantages when usings slots for me is the ability to slot in not only component instances, but any arbitrary figma objects like frames or text.

However, when using the native UI by clicking the plus icon button, it is suddenly only possible to select and slot only component instances:

I’m trying to understand why this behavior happens - is there any good reason for this? Is Figma just not able to search for entire objects on the page because it’s too much?

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What I am particularly trying to achieve is another step towards design-code parity. In code, we use properties. If these properties are empty, then nothing happens and the property itself does not materialize itself in the UI and occupy space.

In Figma you can do something similar by setting either the width or the height of a slot to zero. But then, it’s not possible to use the plus icon button or to drag-and-drop objects into slots. Thus, you have to use the “Add instances” functionality. But there, you are limited to using only component instances, as already mentioned above.