Jaz
Well what makes running bad for the joints (knees) is the force you put on them. That is reduced when the surface you're running on has "give". I also think it would reduce your chances of twisting your knees or ankles in an undesirable way since it's flat. I'm not saying your coach is wrong, but I'm curious to know why. Plus you can watch your form since most gyms have mirrors all over the place.