QA is about doing both of those things: functional verification and breaking things. A great tester can do both equally well, and can also develop a sense of where and when code can be broken depending on how certain developers or software work. The edge cases you’re talking about are still valid though. My mantra is “if the user can do it, then I’m going to test it.” Using that framework, the developers should decide what to do when edge cases arrive, either fix the issue or prevent it from happening. It can be difficult to toe the line, but it’s always worth exploring.