If you change B and the A/B/C/D/E integration test breaks.... duh, it's obvious. Yes, that's easy. But A, B, C, D, E are each going to have their own dependenc

If you change B and the A/B/C/D/E integration test breaks.... duh, it's obvious. Yes, that's easy. But A, B, C, D, E are each going to have their own dependencies. You could change X, Y, and Z and now the A/B/C/D/E integration test breaks. Which one was it? Or maybe you're super cautious and you only change Z. And now the A/B/C/D/E integration test breaks. Obviously it's Z. But how? Why? It's a four-developer team and you're only really familiar with R, S, T, U, V, W, X, Y, and Z. There's also the fact that unit tests serve as living, de facto documentation on how to actually use the classes. You can both use and iterate upon Z a lot faster and more confidently if it has its own tests.