Macs make good development machines if your production machines are also UNIX based. Even if my code is truly platform-independent, I still find it easier when I don't have to mentally switch back and forth between windows and unix. Of course, I also find Unix to be a much more powerful environment. That said, I also haven't found speed to be a problem on my mac (it is slower than the windows laptop that my work gave me, but it's also an older machine).
Of course, this only addresses Mac vs Windows - the argument becomes less compelling if you're comparing it to linux. I'm not sophisticated enough with Unix to appreciate the differences between linux and bsd, but I do like the Mac UI, and unlike windows, I get a unix prompt, which is pretty tremendous.
Because a development platform and deployment platform can be two different platforms.