Neuroticknicky, there are lots of subjects which are never set, such as music, art, drama, and sometimes even history and geography. It's perfectly possible to teach these subjects to mixed ability groups with the right set of skills. Why would it not be possible to teach other subjects in this way?
And as for the ski lesson analogy, well, that's just flawed. Imagine if you turned up to beginner ski school and someone said, "Right, what did your father do for a living? Was he a keen skier? And where did you live as a child? Alpine or non-alpine? Oh look, you knocked a cone over on the test run. Right, you need to go over there with the other adult children of families who never encountered winter sports and came from Norfolk. Here's a trainee ski teacher and some shitty ex-hire skis, to be getting on with. Adult children of people whose parents were skiers and who come from Austria, regardless of whether or not you have skied before, you need to come over here. Meet your ex-Olympic coach. Here are some lovely skis and free lift passes. Off we go up that mountain and we will leave the others in the foothills, because they need remedial teaching and they might be disruptive. Come on, you talented lot, let's turn you into champions".
If that was how you were treated for skiing it would take quite a lot of gumption to break out of the first group and get into the second. Now imagine you are 11, potentially pretty good at maths, and that is how you are treated. It would be very difficult to reach your potential, frankly.
David Hargreaves called this 'A Theory of Typing'. It is harmful and divisive.
I say again, the question is not whether to set, it is how to teach groups so that everyone reaches their full potential. Anything else is a distraction.