How do you mean? The entire iPad? Well this sucks because now you're carrying around an entire laptop and a tablet. Or do you mean using a pen with the Mac? Then it would need fundamental changes to use it well. Using a pen on a traditional laptop sucks more than a touchscreen on a traditional laptop. You need a really really good hinge to have it not be impacted by your arm, or you're forced to awkwardly hover and use minimal pressure. Bare minimum they'd have to add a 180 degree hinge so it can lie flat, or really a 360 one ideally.

With a tablet you get more freedom, not every task needs a real keyboard and sometimes even if it does it's better not attached to the device in wrong orientation.