The problem is that people all too often see the passing tests and think, "Great, I'm done!". They don't take the time to evaluate whether they have thought of all cases.
It's better to have some test cases, rather than no test cases. Even if only the golden path is tested, that's still better than nothing.
I'm merely trying to point out a weakness in the practice of "test driven development", where you write tests before you implement the functionality.
Since you already have tests while you work, it's really easy to just treat them as a checklist, and once all tests pass it feels like you are done.
But in reality, once the feature is done, that's when the hard QA work starts. You need to review the code, make sure you considered all error conditions, check if your assumptions hold throughout the execution of the program, ensure that your code is resilient to unexpected user input, etc.
TDD might be a valuable part of a development/QA workflow, but just introducing TDD on its own is not going to make your product any better.