If the number of cases is small and can be completely enumerated, sure. But there are plenty of algorithms out there where the number of cases is effectively infinite -- the matrix inversion function someone else mentioned is a great example -- where it is prudent to routinely check results to make sure the algorithm is working. (Mind you, also having a unit test to sure it works on a small set of carefully chosen cases is a great idea.)

Sure, but isn't it awesome if, instead of being just one unit tests, you can have some critical checks running in every unit test (and when running the code as a whole)?

However, if you're going this route, it might be worth having a second assert macro that can allow you more fine-grained control of enabling/disabling fast asserts vs slow asserts.