There's really no point considering people using both as anything less than 1 vote for each camp, which I'm sure is what turns up in the data statistically proportional to the average amount of time one is used over the other.
I do find it a little hard to believe that people keep Linux installed "just for the sake of it", if they're never using it then people are liable to remove it.
Just the same, none of this has anything to do with the claims made. Even if a lot of Linux users also use Windows that still doesn't mean that Linux use hasn't grown (especially in the last 10 years, geez), nor does it mean that only 1% of people are using it. Just by looking at the increased number of OEM options regarding Linux you would be hard pressed to think there'd be no growth in the market. 10 years ago who was selling a Linux desktop? Now even Dell, the biggest OEM, is packaging them, and they're available on several netbooks as a cost cutting measure. And the number of advances Linux has made towards being more accessible to the average person are tremendous. You guys do realize 10 years ago was 1999 right? Do you have any idea how much Linux distributions have improved in both functionality and user friendliness since then?
PsyMan, your anecdotal evidence has nothing to do with what I asked for evidence either, that people with Linux are using it more machines/person than Windows. The only real argument to this is price, but your average person won't have a good reason to have more than one computer, and if they do have more than one they'll likely be OEMs that came with Windows, not machines they built themselves and had to purchase an OS for.