The secret behind the P90X system is an advanced training technique called Muscle Confusion™, which accelerates the results process by constantly introducing new moves and routines so your body never plateaus, and you never get bored! Whether you want to get lean, bulk up, or just plain get ripped, there's an endless variety of ways to mix and match the routines to keep you motivated the full 90 days and beyond!
So, is this concept necessary? Um, depends. That might be a pretty underwhelming answer but it's also true because it depends on what the point of your training. If you're training for a specific sport or iron game, or even a very-defined goal, then obviously you need to train with the intent of being good at such game or sport. It's pathetically simple but so many miss it: the only way that you get better at doing something is by practice the something. Strength training is no different. Sure, you may use some other exercises to shore up weakness in your body that are relevant to the primary goal but in the end, the athletic endeavor is the primary focus.
Does that mean that muscle confusion is bullshit too and you shouldn't be changing up your exercises regularly?
If the strange and wonderful strength training universe has a fault its that too many have a far-too-narrow view on what it means to be strong. It's not too hard to fall trap to since defining what is strong is very tricky. I just get the feeling sometimes that they don't try to widen the view though. One lift, or even 5, doesn't define if you're strong or not. Constantly doing the same movements over and over also starts bringing up the injury question. We've all heard of repetition injuries, right? Muscle imbalances? You probably won't get that from switching it up more often. What's the point of putting yourself at risk of either if you don't have a point to do it in the first place?
So, to sum up the way that I feel about this whole question of muscle confusion I think that if you have specific goals, then narrow your exercises down to accomplish those goals. The less specific your training becomes, the more that I think that doing lots of different exercises is worthwhile. I can't comment on Tony Horton's products since I don't own them and I haven't seen the DVD's. I've always liked to do my own thing.
So should you.