For me, the most important point here is that one can often miss the opportunity to verify that one really does understand the dependencies of a system without

For me, the most important point here is that one can often miss the opportunity to verify that one really does understand the dependencies of a system without putting the 'ivory tower' theory into practice. Writing your own replacement for a dependency can help you educate yourself on that layer of the stack, but risks missing the unique nuances of individual pieces of software. Furthermore, debugging specific dependencies allows you to confirm that you really do understand both the subsystem in general and the particular subsystem that a larger system relies upon. Debugging Nginx might improve my understanding of HTTP servers, but that's only half of the story if my system runs on Apache!