edit - also because you no longer have the graphics engineers in house, you are the the mercy of how fast the new ATI company innovates. And you lose the synergy between the CPU/GPU divisions, not to mention the company has to go through another painful shift in direction.
You would be correct pointing out that AMD would be at the mercy of ATI too, but they are already at the mercy of GLF, and at the mercy of Intel, and will be at the mercy of ARM rather sooner than later, so I doubt that it would matter that much be at the mercy of ATI too.
As for those synergies, I don't think those are that critical. In the future, it might be, but what we have on the market today is CPU + GPU, even if they are sharing the same memory space, and this will stick for the next years. All the ARM manufacturers are going for 3rd party IPs for their GPUs, even vanilla ARM is like that. Intel is on the same boat too.
And HSA... HSA is AMD pipedream. AMD is too small to dictate the terms of GPU/CPU integration, they don't have neither the financial muscle nor the market share to push something that big on the market. CPU/GPU integration will be dictated by Intel, Qualcomm, Samsung and Apple, not by AMD.
The synergies you are stating simply doesn't exist, or at least they aren't relevant enough. What AMD has today is simply a luxury IP maker that allows better control in their development pipeline, but every other player on the market is doing better than AMD, and except for Nvidia they all license GPU IP.