Kinda hard to make a comparison considering the different platforms.
An all around professional grade PC is a desktop, so it makes sense to be able to support everything in your workspace. Optical media included. My father is an architect and he has a rather impressive library of CAD resources on CDs and DVDs. Despite going digital, they still walk around with rolled up blueprints. His drawing table (it was a nice table) is put away in the storage shed tho. I remember that it was a while before they stopped using floppies. In a sense, mine's more powerful, but his PC is more decked out than mine.
A professional grade Mac is a laptop. Which, while already several times more expensive than a decked out work PC, is going to have even more components removed?
I've always thought of a professional computer to strive for compatibility with your existing workflow. Your tools are supposed to help you with your work, not be dictated by it. A USB drive that you can leave at home? For a casual user, yes. But not for a professional.