Gore even if you start out at square one, meaning a new company and new people and use Linux from the get go, I don't agree that a lower TCO is valid. Users can't even use Windows without causing serious problems. I would argue the network would be unstable and shotty not because Linux bad, it's in many ways a better OS, but because users won't figure it out without a large and costly Linux team to hold their hand.
I was reading up on something where admins smuggle in Linux and BSD boxes, and just don't tell anyone that they did it because they are supposed to use Windows. I'll see if I can find it.
You are exactly right because Linux provides for Fun in a way that spurs curiosity. It's free and you can load anything you want. That doesn't equate to lower TCO across the board. For instance, you may have a need for some security scanning and the boss won't pay 2000 for Languard, so the curious and energetic admin loads Redhat and follows a howto on setting up a Retina scanner. He's happy, the boss is happy and in this case the coin is flipped. It's common and perhaps very good for both the admin and the network, assuming he isn't smuggling in his own warez box to knock out some critical component.