I used to be a chef. Don't order anything above medium.
Firstly, they can't cook it fast enough so that it comes out with the rest of the meals, thereby upsetting the customer, the waiter, the chef and the rest of the dining party. Secondly, over-cooking destroys good steak. At home, with low grade meat, fine. But good porterhouse, T-bone or other good cuts of steak just doesn't need to be cooked too long. It destroys the texture and the flavour.
Thirdly, steak done this way can be a little different to the palate - honestly though, do you go to a restaurant to have exactly the same kind of meat you can do for yourself at home? Be adventurous!