To a degree the Met CD will add to the ATPL theory, but should not wholly be used as a replacement. Met is one exam where you will need to learn concepts and be able to draw the picture on the paper to gain the correct answer. With the diagrams and animations on the CD, you should be able to build up a picture of what is going on and apply that to any question by drawing your own sketch. For example, being able to re-produce the CD's jetstream diagram will allow you to answer which way the core is flowing, where the warm/cold air boundaries are and be able to assign heights to each.