As a personal trainer, I think that changing it up every workout is important for the everyday lifter, who just wants to stay in shape and be healthy. People often tell me that "going to the gym gets so boring." Therefore, it's important to change it up at least to keep people motivated, as doing the same thing over and over again will get boring to the average gym rat.
However, the muscle confusion principle is likely a load of crap if you're an elite athlete or competitor, etc.