I like the SQL/CSS analogy, with the full-stack spectrum being like:

  SQL - Backend - Frontend - CSS 

Most people just want to stay in the middle doing backend/frontend ("full-stack") work because it's just writing functions. Or because it's building the core of a feature, where things like database work and styling is pushed off or abstracted in libraries.

But on each end is where all the interesting stuff is really happening, and a full-stack dev who embraces both SQL and CSS is a lot more useful than a full-stack dev who stays in the middle.