Although I can see this giving some flexibility in how a "single" post is displayed, for example, a portfolio item, I'm wondering how this will affect the user if they later change themes.

What I would LOVE to see the core team create is the ability to choose blog layouts by way of "page templates" or rather "blog templates". Give the ability to have various blog layouts (incl. archives such as categories) from a template choice from the customizer. Just like choosing a page template for a page, this would be done the same way for blog layouts, like a grid layout, masonry, right sidebar, left, full width...etc. Right now themes have to create multiple files and conditionals, if/else, etc etc to achieve that.