I guess one can't expect any camera to beat the competition on all fronts. Each camera has its strengths and weaknesses. I consider the sensor based stabilisation in the A900 a major strength, especially for people like me who already own a few Minolta-fit lenses with no optical IS.
On the other hand, the significantly lower DR of the A900 at high ISO (compared with the 5D2) is a bit off-putting, as well as the lack of a Live View.
One really has to weigh the relative significance of such strengths and weaknesses. One third of a stop higher DR of the A900 at ISO 100, compared with one and one 3rd stops higher DR of the 5D2 at ISO 1600 and above.
One amazing feature of the Canon 50D with its high resolution Live View screen, is just how much detail one can see through a 400mm lens when the image is magnified 10x. Another amazing effect is just how unstable and wobbly that image can be, even with camera on a tripod, without in-lens image stabilisation. With no magnification (basically the same FoV as one sees through the viewfinder) the image appears stable when camera is on tripod without IS activated.
However, at 10x magnification, which is like looking through a 4,000mm lens, the image wobbles with the slightest breeze or vibration. Image stabilisation is essential for critical manual focussing in such circumstances.
Designing a full frame sensor with anti-vibration was a major technological challenge for Sony. To also provide an image stabilised Live View feature was just too difficult or too expensive, I guess.