Joel Spolsky's latest Strategy Letter does a nice job of explaining why religious wars miss the whole point of open source. Related products, like automobiles and gasoline, are "complementary"; their sales are related. If cars get cheaper, more gas gets sold. If gas prices go down, people tend to buy bigger cars. Computers and software are often complements.
A British scientist at Hypertext '02 was surprised to find out that Eastgate was behind Tinderbox. "I thought is was another clever way Apple had found to sell more Macs!", he said. At that very moment, one of his colleagues walked up and mentioned that he'd finally convinced their department to sanction a new Macintosh for Tinderbox.
Of course, Tinderbox for Windows is under development. But sometimes it's better to get started right away -- and computer pros need access to both platforms in any case.