The really compelling applications are at a business level - ERP, CRM, finance, engineering etc. - and might not come from Microsoft.
There is a huge world of specialised business applications out there that are ignored on HN (often for good reasons) that any large enterprise will have hundreds of - a lot of these run on Windows.
Yes, there are many business applications, that run o Windows.
However, there are two trends now:
1) Move the application to the web. Many of them are getting HTML frontends, and the backends are OS-independent.
2) Those, that are not getting ported, are being thrown into Citrix XenApp (or into other desktop virtualization environment). It is actually very interesting to see using your business app on device like Asus Transformer. And then you start question yourself - why do you need Windows on client and all the heavy infrastructure in the server room it needs?
I was only addressing why people want Window Server - clients are something else entirely. In my experience, in large enterprises almost all "business" applications (except for Office) are served over Citrix or RDP.
There is a huge world of specialised business applications out there that are ignored on HN (often for good reasons) that any large enterprise will have hundreds of - a lot of these run on Windows.