A quick take on a controversial UI design practice.
What happens when there is a significant change to the components? IE: The header is now blue vs white. Then you're going to need to go back and update every screenshot to reflect that change rather than a simple update through the library. I have experienced designers who work with screenshots and the files are large, theres inconsistencies, and it makes it a pain to update any new UI standards.
Designing with screenshots might be an obvious choice for some but you really drove the point home :D