With the new Apple laptops they just apply a 2x scaling factor globally, which means that button images resize without getting ugly. On the 15-inch model the GPU then scales *down* by 0.75 to fit the display. A neat trick, but it wouldn't be necessary if the software were able to change the dpi setting properly.
Even if you have perfect eyesight, you don't want to view a 300ppi display with glyphs designed for 100ppi. My vision (after surgery) is better than average and my monitor is only 200ppi, but it still looks unreasonably tiny if you don't increase the font size.