Blizzard puts up public test realms (PTRs) for testing new content all the time. At times the PTRs are unplayable, either due to bugs or overcrowding. At times they are ridiculous because of the economy, because people transfer all the gold on their account to a single character before copying them. Some major guilds go whenever there is new raid content to practice it on the PTR, so they can get their world firsts whenever the content does go live. Some people seem to go just to stand around and complain about the changes yet not actually test anything. However, all the problems aside, the PTRs are the only way to experience the new content or class changes prior to live.
I tend to copy over to the PTR whenever there is a new raid instance or a major revamp of my class. Having been in the betas for the last couple of expansions, I tend to file lots of bug reports and hope that at least some of my issues are addressed. I have gotten frustrated with people attempting to make a ton of gold in the test realms because they do not seem to grasp the fact that the money doesn't come with them to live. Whatever is done on the test realm stays there and dies there.
The PTR is there for testing, and testing requires people to be able to have access to the new material. Because a lot of people don't actually test on the PTRs, we tend to have several mini-patches in order to fix bugs with a major patch. So actually use the PTR for testing, since it will only help cut out some bugs for when it goes live.
Do you actually use the PTRs? Do you participate in sending in bug reports? Are you one of these people who copy over all your gold and charge exorbitant AH prices on the PTR?