With Isolator, verifying method calls is less brittle. Even if your code calls a method mutiple times, you can verify that it was called at least once with the correct parameters, regardless of call order, and ignoring all the other calls. This is called "Relaxed" or "Non-Strict" behavior, and some Isolation frameworks are strict by default. 

With non strict, if your production code changes to add more method calls, your test won't break. Your tests are less brittle.