> Testing more than one class at the same time doesn't make it an integration test. Arbitrarily restricting a unit to map to a single method or a class is a good way to ensure that your test code is tightly coupled to the implementation.

But at least if you restrict your units to a single method, you have a chance of getting somewhat complete tests. If you're testing multiple classes with several methods each as a unit, the number of possible code paths is so huge that you know you cannot possibly test more than a small part of the possibilities.