Everyone looks at it from a safety aspect. Even IF a primer got struck, hard enough to dent the primer cup, and it went off, you still don't have a shooting bullet here. Absent a chamber to contain it, the the brass case is going to pop off or blow out long before any dangerous pressure is built.
What about looking at it from a functionality aspect. My last batch of 9mm used remington primers. 3 of them have failed to fire, and with good, solid firing pin strikes. I have no proof one way or the other, but I'm beginning to wonder if tumbling could potentially reduce the reliability of primers. (just an idea...)