In swtor, they had this, along with a function to copy your character to the test server, eventually, so you could immediately test new content. It worked out great. While I am sure many just used it to try new content sooner it did result in better released content. I can't say how many in-game tickets were filed, but the test server forums were very active with both bug reports and general feedback. I only participated in the test server for a couple major updates, but I can say that for those I did participate in, the released update was far better than what I tested. Having 10-30k people testing for a few weeks in invaluable even if only a small percentage are being diligent with reports.