Serious Linux users use it because it's the only thing that fits the way they work. Linux has all the interface and programs I'm used to, so of course I'd be more likely to buy software for it than any other OS. What holds me back is that there isn't any software I need that I don't have...
One problem that I do have is that if I pay Canonical, for example, to make the whole OS better, it would mostly still crap out because I made the mistake of buying at AMD/ATI graphics card. Not sure who to blame at that point. The Ubuntu people say its from ATI/AMD and ATI/AMD say that there's not enough money in the market to justify better work on the driver(that or technical problems with the differences between distros). So what then? It would still have the same result.
But, in general, I do vote with my dollars.