Based on your argument, I can see why you'd think this. Your argument is one that I would expect from an audiophile, but not a full time musician. The lack of an option to mix on headphones, is unacceptable to many musician, ESPECIALLY these days.
When you're in a profession studio and see a producer come in for a session and place his Macbook on a mixing board, for a quick session and then leave, you'll get it. Most musicians work on the go, when working in the studio isn't an option, for whatever reason. Nobody wants to carry around interfaces.
I'm not trying to argue a point due to my desire for a headphone jack. It's simply how it is. As a full time musician, it's my experience that colleagues and other fellow musician acquaintances are increasingly producing on the go. That's why I know you can take a headphone jack out of any other Mac product, except the MBP line.