Well, quite simply put, if you have safari and iOS in your support matrix, how else are you going to debug for it? No other options than using a Mac(book).
The popularity of iOS and now OS X has forced developers into Apples ecosytem. I must say I don’t mind developing on a macbook pro retina though, previous dev machines were all linux and windows, but this… is so much better (also a lot more costly of course but well..).
Next computer that will enter my home will be a Mac, no doubt. I’m done with self built windows / linux boxes.