I think the key difference is an engineering mindset. When you have software developers skilled in quality assurance who have the job of finding the edge cases

I think the key difference is an engineering mindset. When you have software developers skilled in quality assurance who have the job of finding the edge cases and producing comprehensive additional acceptance and functional testing, they're an asset. It's a particular perverse mindset that I personally really enjoy interacting with as a software developer 'customer' - those evil bastards find the best bugs, regression test them, and expand to find all of that class of error in the application.