Is stable, so I have the confidence that in 3 or 5 years time, the same applications will work.
"Stable" can mean a few things, but it's certainly not "stable" by your definition. Tell that to all the people who won't migrate from XP to Vista, because their applications won't function properly under Vista
Actually, the same is true for Linux. glibc anyone? Older applications don't always work on newer distros. It's just that a lot of stuff is rebuilt in the distros repos. It's not a failing of stability, it's more an advantage of the package manager systems then anything.
Is integrated - so I can work quickly and efficiently.
I have no idea what this means, and I suspect I'm not alone. Next "point".
Actually, I think I can answer this. At home I run Linux and at work we have Windows XP. Can you say, with a straight face, that Linux has anything close to what Outlook+Office+exchange allows you to do (and does it as well)? Yes, there are things that we can do that are "equivalent" but with Windows having it all integrated is a time-saver. Heck, Outlook 2007 uses Word as a default editor. Can you say the same thing about Evolution or Thunderbird or KDE's equivalent? Outlook's calendar is very easy to read whereas evolution's display (or at least its default one) looks like crap. The point is there's tons of "little" things that add up.
That said, I will always have Linux on my personal computer(s). But honestly, using Linux would likely make my company's employees less productive just by not having the Office suite available. That productiveness makes up for the cost of Windows when you break down the time saved. Until there's something available on Linux (or some distribution) that is at least as good as the Office suite then it's not worth it to switch.