From a manufacturing view point (of which I am intimately aquainted), it is much easier to set up the machinery to run one part, than to run a number of parts. A good move on Microsoft's part if you think of millions of that one part being manufactured. They will probably save millions of dollars in manufacturing costs during the full life-cycle of Vista DVD production.
As for whether the OS keys can be hacked easier on one disc - I doubt any method of manufacturing is going to stop the inevitable pirating of the software. I am sure Microsoft is fully aware of this fact due to their being in business for quite a while now. Assuming they have a bunch of unintelligent people working for them is unrealistic.
This would seem to provide some relief from the cost of production vs. the cost of lost revenue due to pirates, as well as providing an easy upgrade to potential customers.
Personally, I could give a rip about pirates. I will purchase the new OS as soon as it becomes available simply because I have tested both my notebook (2005 model) and newly custom built desktop (Sept. 2006) with the RC1 and have found a lot to look forward to. RC1 ran fine on both machines, and assuming the third party hardware/software providers offer updated drivers and software support for Vista, life should be good.
It seems to me that Microsoft has been paying attention to the graphic's part of the customization community (shown in their use of reflections, glass effects, and overall improvement in the graphic quality), and that it will shortly be upon the third party developers to deliver the proper drivers and update patches so that users do not have to purchase all new hardware and software to use the new OS.
