I imagine stretching tires as doing several things. The bad would include making the tire more vonerable to popping a bead as well as less contact area (stay with me). Tires are designed to have a stiff corner (where the tread meets the sidewall) Think of an 'L' shape. Now when you bend the sidewall in, it also bends where the leg of the tread portion is pointing (essentially pointing the leg towards the center of the rim). This makes for uneven distribution along the tread (greater wear on the outside). I'm sure this could be compensated with a lot of air pressure. But this would harden the tire up like no other (why they do this for drifting). This is just the way I interpret it.