Also, another thing. Linux/BSD push the bar way high up and force Windows to try and catch up. So even if you use windows, you're still affected by Linux. In the same way that you're affected (positively) by Apple even if you use Ubuntu.
The phrase actually means that the existence of a specific rule implies that the negation is true for circumstances not covered by the rule. So, if half of the blocks in a city have "parking prohibited" signs, and no such signs elsewhere, that "proves" that parking is in fact allowed on the other blocks, since the signs are excepting the general rule. In other words, why would there be a sign if it was understood that there is no parking?
Sorry to make such a big point out of a small matter, but we already lost one good phrase ("Begging the question") to widespread improper usage. Wikipedia has more info, including the origin of the phrase: http://en.wikipedia.org/wiki/Exception_that_proves_the_rule
Put another way, when used correctly, "the exception that proves the rule" is a valid rhetorical phrase regardless of how many exceptions there are. Whereas if there were 500 successful startups using non-open-source tools, you wouldn't say that they were the exceptions that proved the rule.
But you do. That's the point. It's not an absolute law. I'm sure you can find a great application written in Visual Basic. This doesn't make Visual Basic a great choice over Python.
If you could list 50+ successful startups running on Windows, then you could say there's a strong evidence.
And even then, this only proves that Windows is as good as Linux. It doesn't counter the original point about Linux being great despite not having a business model centered around IP.
Edit: For example, could you write CAD software without an OS as we know it today? It would be undoubtedly harder, but you could. People did this in the 60s.
Since people lived just fine before technology, we don't need technology all that much, therefore we don't need IP.
The older AutoCADs (on DOS) apparently plotted the internal grid (hi-res), then resampled it to generate the screen display.
Thus, minor panning and zooming could be accomodated without "regenerating" -- replotting the hi-res grid.
Anyway, a CAD system doesn't really use the facilities of an OS; something like a web server, however, seems to benefit from an OS.