Very true Nadrek.

I worked in game development for 7+ years. It was always critical to have QA involved in the reviews. Maybe not every tester, but leadership present is valuable. Like Nadrek mentioned, they are as close to the end user as possible. It wil save a lot of time in the long run when you start including them more rather than blocking them. That's because you are one piece, not the entire pie.

Overall, you should not fear non-developers/designers from dictating your outcome. They are there for influence to help you take the right path with your changes. You can either chose to accept feedback or ignore it. The choice is still up to you.