Most of the systems I've seen over the last few years have been written in layers. Down the bottom, you have components that do the important stuff, but don't talk to other components much. The layer above coordinates between the lower-level components, but doesn't do much interesting itself apart from that delegation.
So what happens if you test those higher-level components with mock objects? Well, you sort of end up with tests that do absolutely nothing but test a series of method calls on mock objects. This is, I would suggest, worse than useless. What are you testing, really? You're not putting forward any kind of verifiable hypothesis about what the method you're testing should be producing: you're just writing it twice, once in the test and once in the object being tested.
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