Honestly I'd rather have all the units covered in isolation, and leave it up to my manual testing to catch any unexpected errors or bugs, then let our QA team do the same with their own Selenium based black-box testing.

If the contract between two units is flawed, you only have to worry about updating the behavior of one or both units. The unit tests make sure they work as designed.

It seems to abstract the problem for me to the level of only detecting missed integration issues. I don't need to worry about how those units do what they do, just that they do what they're designed, because the unit tests say so.

There's nothing I hate more than making a small change, and having to spend hours tracking down why 20+ tests that are written as integration tests rather than unit tests (with mocking).

I'd rather maintain the mocks when making changes, than have to spend all that time trying to figure out what complicated and huge, far reaching, thing is broken.