Your 15 - 30% using alternatives could well be right for the world at large.
For many people OS X and associated apps is all that is required. For many common apps, such as Office, there are OS X and Windows versions available. Material created can be used with either OS. However there are a few apps that are still Windows only, thus necessitating its installation to use them on a Mac.
The cricket scoring app we use for the local league is an example. When I have used it, I do so on a Windows computer, but some friends do have it on their Mac.
Within this MacRumours community I am not surprised to see the figure up at around 50% using alternatives to OS X. There is a large contingent here who like mucking around with computers, including checking out and using other OS.