Perhaps the best argument Microsoft has so far is that Vista’s and Windows 7’s kernels are very similar and thus, the transition from one to the other would go much smoother:
“It is a goal of the Windows 7 release to minimize application compatibility for customers who have deployed Windows Vista since there was considerable kernel and device level innovation in Windows Vista. The Windows 7 release is expected to have only minor changes in these areas. Customers who are still using Windows XP when Windows 7 releases will have a similar application compatibility experience moving to Windows 7 as exists moving to Windows Vista from Windows XP.”
So, basically, the paper says it’s better to have a head ache now than later. Of course, we’re talking about a smaller head ache now, given that software has been polished to run properly on the new OS.
However, it’s unlikely that this would actually make a difference and persuade businesses to adopt Vista. As the old saying goes “If it works, don’t fix it”, and Windows XP works just fine right now.
"The Business Value of Windows Vista: Five reasons to deploy now" (PDF) is available here.