@BNP
Possibly because they need to use Outlook (at least) and the rest of the Office tools in order to do their jobs?
I have a personal MacBook and a company Windows Laptop. Can't remember the last time I booted up the company machine to be honest. I have Windows installed on a parallels install that has Office and the server apps I need to run, but to be honest, if all I'm wanting to do is email or work on some documents then its a royal pain in the ass to have to start the Windows partition and then wait to launch the apps (and the subsequent performance hit due to running in a VM).
... and yes, I've tried iWork and I have OpenOffice installed, but to be honest, they just don't really cut it....