This is a good question. I use Macs for web development because they're built and designed extremely well, and because OS X is UNIX based. IMO with the MBP Apple has found the perfect balance of power, battery, display, thinness, etc. Apples trackpads are still second to none. More important to me is the fact that OS X is based on UNIX (FreeBSD specifically) which means that Apache and PHP run natively in a Linux-like environment. It's much easier for me to predict how code will run on our Linux based servers when developing on OS X. IMO OS X's interface is also much better. There's no weird mix of modern and desktop UIs, no registry, no drivers to mess with, etc. OS X lets me get straight to work and not worry about maintenance - very important when working with a deadline.
But mainly you should use what works for you. IMO my MacBook was an expensive investment that pays off by allowing me to work much more efficiently and when I'm out of the house. This doesn't mean you can't necessarily accomplish the same things with a Windows PC but Macs just fit my needs much better