Any business whose standard is Windows XP will be in for a surprise when the support is dropped very soon!
I work for a financial company that moves billions of pounds every day, and we use a lot of free and open source software. For some mission critical machines we have a support contract with Redhat (even though the product is open source). For other machines we use CentOS, which is basically the same thing. Linux isn't confined to our servers - we have it on the desktop too with full AD integration, despite Microsoft's attempts to break interoperability at every turn.
We use open source software on these machines as it is leaner, much more customisable and deemed by us to be more stable, secure and easier to maintain, with smaller overheads.There's no faff of maintaining records of licenses, no need to learn the latest Microsoft silly wizards and you can learn how things work rather than having a black box of magic. No vendor lock in is good too.
Of course, there's always things you need Windows for, so we have Windows 7 desktops and some Windows servers too, but they are in the minority.
I won't argue against the fact that Microsoft has the typical office products nailed and is way ahead of the competition (OpenOffice/LibreOffice sadly don't even come close). Office is Microsoft's heartland, where they perform best. However, if you are a company above a certain size who is going to need an IT department anyway, and develop a lot of highly mission-critical software inhouse that has to be reliable and error-free*, open source platforms are a very viable option. My opinion is that FOSS is the best option for this.
For a smaller company, or a company who only uses their IT systems for email, spreadsheets and word documents, perhaps the Microsoft way is best, as employees are more likely to be familiar with it and supporting staff easier (and cheaper) to find.
I run FOSS at home, as that's what I'm used to, and I like tinkering with the internals of my systems. That isn't for everyone I know, but I wouldn't have it any other way
FOSS isn't just for cheapskates, it's also for those who want reliable systems and visibility into their workings with no faffy wizards. The fact it's free moneywise is a bonus.