This is the critical distinction. I think a lot of people see Sketch and think "so, a cheaper Adobe Illustrator?" But it's how Sketch utilizes nested symbols and libraries that makes it a qualitatively different kind of application from other vector apps.
I love that you used the term DRY to describe the design process Sketch enables - it never occurred to me, but it's exactly the same principle. Going back to Illustrator or Photoshop after Sketch is the UI design equivalent of copying and pasting the same code block over and over instead of writing a function or a class.
