To me the question was not 'why use a mac', but rather 'why not'.
A couple of years ago I was trying to make a decision between an HP EliteBook and a Lenovo W530. My only real requirement was a high-res 15" IPS screen, and both HP and Lenovo had a suitable config available at around 2200. At some point I decided to check what Apple had to offer, and noticed that the (then brand new) mid-2012 rMBP was available with an almost identical config as the HP and Lenovo, but came in a much slimmer and lighter package, with much better battery life, and a higher screen resolution. And even better: at the same price.
Most of the software I needed was already available for OS X, so I figured, why the heck not.
Needless to say, I've been quite pleased with the choice.
HP's Ultrabook Folios are very nice from a 'fit and finish' perspective. A few of us at my office have just taken delivery of some. It's pretty nice to not have to lug brick around from my desk to conference rooms, etc a few time a day.
Ultimately, it's a matter of the windows user experience versus OS X that makes people choose Mac over Windows. I'd be remiss to not say that the windows experience has improved dramatically over the past 10 years, though I just won't give up my OS X experience in 10% of my day for the windows experience I use for the other 90%