Yes. The best QA people are gold. Infuriating at times, but gold.

> end up having the most knowledge about the application works in practice

The best I've worked with had this quality, and were fearless advocates for the end-user. They kept everyone honest.

I was at a company once where they were talking about trying to do a rewrite of an existing tool because the original engineers were gone. But the requirements docs were insufficient to reach feature parity, so they weren’t sure how to proceed. Once I got the QA lead talking they realized he had the entire spec in his head. Complete with corner cases.