I interpreted the parent's usage of "reliability" to refer to engineering-lifecycle reliability. I completely agree that confidence in tests is critical, but I also agree that taking design shortcuts here and there in test code is fine, if it gets the job done and doesn't adversely impact the production API.
It is one of those fuzzy, learn from experience type of concepts.
In my experience, taking shortcuts in tests breeds more and more shortcuts (as less experienced developers look to existing tests as reference). As the shortcuts accumulate, the tests become more brittle, less effective and harder to maintain. This is the path to having a test suite that you no longer trust.
