so I suppose internal alpha/beta testing doesn't give them any insight of how the finished product will be judged by the masses
When you're on the inside, and when you've been working on something for so long, you can lose sight of how outsiders will perceive the finished product. It takes fresh eyes to look at it at that point.
Your point seems to be coming from the perspective that the game is bad to begin with, when, I'm sure for Quantic Dream, they thought that their work on Heavy Rain was really good, and they were proud of it (as they should be, in my opinion).
It's not until some time has passed, and they've maybe moved on to a new project, or has managed to get some time away from it (a few months at least), that they can then go back to it, evaluate it, and take notes on how to improve it.
For their first PS3 game, I think Heavy Rain was excellent. In my opinion, it can only get better from here, if they continue to strive for improvement. It's not like David Cage has said, "Heavy Rain is the pinnacle of this kind of game, and there is no place to go from here." He appears to recognize the flaws of the design and narrative, and are working with his team to improve them. Why this is grounds for the typical Neo-GAF David Cage hate brigade to once again trash him is unclear to me.
I don't like Cliffy B, so I stay away from his games, not go to every thread about him and stink up the place (not saying you are, but many familiar faces in this thread have shown up in all the other Quantic Dream/Heavy Rain/David Cage threads and spout the same stuff each time).
Cage is certainly arrogant, but his goals and intentions for his games are reasonable and admirable. It's not like he's trying to push every single video game in existence to be like his games. There is plenty other approaches to game design that are more traditional than his. I don't see what the problem is.