WHEN to use Partial for Banners, if ever: Deciding Factors

I’m just wondering if there’s ever a case to be made for using PARTIALS in banners.

There is the dropzone for alternate banners in RW.
When are those most sensible choices?

I’m inclined to think that dropzone would be used only if you are making a theme variant for a specific page, different than whatever you are using as dominant theme in your project.

Where’s the Break Point
Does it EVER make sense to drive a 2nd or 3rd theme in a project, simply for a new banner?