Okay, please show me the evidence to support this...other than "it doesn't make sense". Once again, you are showing a lack of understanding of marketing factors and manufacturing process. First, to prove my point, I did a quick google search and this was the first article that popped up.
http://www.fudzilla.com/content/view/15828/35/
Follow the link and translate the webpage. This was done shortly after the launch, with the team collecting 120 random chips and attempting to unlock them. They had a 73% success rate of fully unlocked and 100% stable chips (ie chips with NOTHING wrong with the locked cores). It should be noted since this test was done the success rate has gone up because AMD had a backlog of chips that did have bad cores at the beginning that is now gone.
They are filling a market segment. AMD has NO shortage of quad core processors. They cannot sell all they produce as it is. So what should they do...cut manufacturing? That makes no sense because of the fixed cost of manufacturing.
So they sell the excess chips as X2 and X3 models to attract a different market segment. True, they don't make as much on these as the quads, but they do make some money on them. Something is better than nothing. It is a simple and effective marketing strategy to maximize the yield from manufacturing. Fewer wasted chips means more profit overall, even if some of those chips have to be sold off at a $25 - $50 discount to their quad core counterparts.
Here are some more links that show there is a large success rate getting a 100% stable quad core from a x2 or x3
http://www.mymobile88.com/success-unlock-amd-phenom-ii-550be-cores-why-amd-lock-hide-phenom-ii-550be-720be-cores/
please notice the last paragraph
"AMD reported record increment in sales of AMD Phenom II processors, no doubt, in part due to fourth core unlocking. Many believe that the increasing rates of successful unlocks is due to AMD purposedly binning perfectly working X4s as X3s to fulfill market demands. This mean your chances of getting an unlockable AMD Phenom II 50BE / 720BE chip just got better. "
http://www.xtremesystems.org/forums/showthread.php?t=227004
Keep in mind many of these fail because of motherboards not fully supporting unlocking, not always because of the cpu itself.
I could go on listing links all day, but you can do the searching yourself. The polls show between 70 -90% success rate. Almost every hardware site has successfully unlocked the cores and ran test showing them as fully functioning 100% stable quads. You can believe what you want based on "it doesn't make sense", but the facts support what I stated in my previous post