Since most of the balancing changes seem to assume you have the right trinkets and top-end gear, no real point in most of us coming to test changes. Hate gets nerfed to balance trinkets I don't own. HARP gets nerfed because of weapons I don't have. Am I supposed to come to PTR and grind out the honor to get those weapons to test a change? How many people are going to do that instead of developing their character on the live server?
Getting a premade rogue let me get on and actually test changes as Blizzard balances the top end. I could try the different changes to the different weapon specs, or see how the changes affected my class on an even playing field with the other premade people. It also brought more people to the PTR (important when wanting to test changes to group content).
I can understand not wanting people to come try premades for classes they don't play for new content testing, but ruling it out completely? It's not like most of the players are ready for Sunwell right now. You will get minimal testing, and testing reports from people way undergeared for it.
No, they'll get feedback from people who know how to do instances, eg BT raiders. What good does some scrub in greens, getting a premade do? He'll run in cry nerf and that will be that.