The problem with shifting is that you are crossing your feet, which is basically something you are taught not to do the first day (ok maybe second) of doing footwork. For a brief moment your legs don't have proper positioning and you are squared up. If you get caught during that shift you will easily get hit with a flash knockdown. Just a very aggressive move that risks defense for a positive in offense. People still do it. Not trying to bring MMA into the scene but Machida does it a lot, since it's pretty standard in Karate.