I think it's a mistake to assume that integration tests will capture all of the bugs unit tests will. Even with a full whitebox approach, the analogy I'd make w

I think it's a mistake to assume that integration tests will capture all of the bugs unit tests will. Even with a full whitebox approach, the analogy I'd make would be testing from the next room over with a waldo arm that can only move in a preset number of directions. It's just much harder to put together tests that hit any level of internal granularity than it is by controlling closer to the target. Theoretically it can be done, but that doesn't make it a good idea. Plus you potentially end up with a large body of brittle tests that can be invalidated by any hiccups in the stack. Integration tests really should concentrate on the behavior of the integration. It's not just a philosophical approach, it's about not hitting the hockey stick of diminishing returns in your testing while trying to maximize value of the results.