I've decided we should kick all the testers out of sprint retrospectives. We don't need them. They aren't really part of the team, anyway. They think differently and their issues aren't really the same issues faced by the real team.
How'd that sound? It was hard to even type so hopefully it took you pretty far aback.
However, I occasionally hear exactly that same message except about product owners. I can't think of a single good reason why a team would want to hold a retrospective without the product owner present. I can think of many bad reasons why they may want to--but each of those should be the topic of a retrospective!
If you're tempted to run a sprint retrospective without your product owner, I strongly suggest you think about why. Product owners are full, first-class team members. It's critical that they participate in retrospectives and they are as open as everyone else to hearing things they can do to improve. Teams that don't include their product owner tend to suffer from us vs. them thinking that is almost always harmful to the project.
I hope your next retrospective is successful. To help that happen, make sure the product owner is there.