It doesn't have to be abolished, just use a modular one that can be easily slid out to take a second battery. Pc laptops have had the feature on some models for ages. I think it's a great idea.
Yes, I agree that is a good idea. Also, you're exactly right; PC model's have that feature.
However, If Apple does that. Then there is problem of having a less "clam-shell" like Laptop. That's what I really like about the Apple Laptops. They're very sleek with out any edges, bumps, slots, etc... If you put removable devices in then you'll lose structure strength