Suppose 90% of people use a PC, 5% use a Mac and 5% can use both. Because such a large minority can only use a Mac, every store and organization, especially schools and hospitals, have to run two support offices and publish instructions for every operation with both PC and Mac instructions. Everyone can see that if everyone used a PC society could save money, but no one knows how much.
So the question is, why not force everyone to use a PC?
1. Some people grew up using a Mac and it might be hard to learn to use a PC.
2. Maybe it's very expensive to teach people how to use a PC.
3. Some people might have good reason they prefer the Mac, enough to justify the negative externalities on society.
This is a bit of a stretch--it's not that hard to teach people to use a PC and Mac's and PC's have considerable interoperability. You can network them to each other, for instance.
But consider substituting "English" for PC and "Spanish" for Mac. Now how true is this story? Is the US stuck in a suboptimal two-language equilibrium? How large are the negative externalities to learning Spanish? I bet someone has written a paper on this with a wildly inaccurate estimate.