I don't think anyone will buy a Mac just to run Windows. But even if you like Macs, you may need to run some Windows applications, and although sometimes you can get away with VirtualPC or similar, other times -- and here I'm thinking of hardcore games -- that simply won't do.
It would make testing apps across platforms a *lot* more convenient, probably cheaper too if you otherwise had to buy another machine. Does 10 gig seem like a lot of space?