Correct SSS is not so slow to render, but it's complex to developp. Which is why it's much easier to do a simple "trick" and call it SSS. The "big deal" is that to render skin, marble and other SSS materials correctly, you do need true SSS. And you don't need such a trained eye to spot the difference. One simply looks a lot better than the other! And if you do animation, the difference simply becomes blatant!