In reply to Matt.

And it will make it almost impossible for me as the designer to control the design of the website if clients start laying out and designing every page themselves.

This is actually one of the areas I am more excited about Gutenberg. I think it gives more tools for designers to reliably specify the look of a website while allowing their users to comfortably edit pieces of it, or change the order of things if allowed.

Consider one of those ubiquitous three column "feature" grids many websites have—with an image, heading, and some text for each item.

Right now this is not trivial to make. A site developer has to construct complex markup and then consider how their users may be able to edit this without ruining the layout. This often leads to having a somewhat convoluted setup where you assign a page, and indirectly tell users that they need to edit this page over here to have it affect the text shown over there. Or perhaps it's a shortcode with different fields for each column, heading, and image, that is displayed in a modal, and doesn't look like the final result at all.

With blocks a developer would be able to provide a theme-specific block that directly renders this layout and clearly specifies what can be directly edited by the user. That means the user gets a WYSIWYG experience where they can't mess up the markup but can easily update text, swap the images, reduce the number of columns, etc, without having to ask the developer for it and without fearing that they will break things.