No, because then the user of the custom element has to put slot attributes on the things they put in the tabs. Consider this:

    <tab-area>
      <tab- text='First'>
        ...arbitrary HTML that should work intuitively, i.e. be styled
        like the rest of the document.
      </tab->

      <tab- text='Second'>
        Note the lack of slot attributes on these tab elements. That's
        because we want this to behave like normal HTML where you
        can nest without having to worry about how tab-area was implemented.
        Why should we have to care that tab-area was implemented with
        templates/slots?
      </tab->

      <tab- text='Third'>
        There could be any number of these tabs so you can't put them
        into slots without some sort of late-generating the slots anyway.
      </tab->
    </tab-area>

We want this to render like a group of tabs where "First" and its body are visible, and tabs are peeking up from behind with the texts "Second" and "Third" but the content of those tabs isn't shown until you click the tab (at which point the content of "First" is hidden).

That's not achievable with slots.

In general, I haven't found a case where templates/slots are actually useful--it's a lot of work to create a bad interface.