So how does that effect opposite muscles. I've done this since I started lifting and I've gained muscle and strength, and have never said my chest is sore today so I'm not gonna work legs.
I look at it like this (and please correct me if I'm wrong);
You have a limited amount of energy you can store in your body -
like a health meter on a video game. When you train, you cut into this reserve.
So when you get out of the gym (monday) your energy level is at 50% from
exhausting your legs from leg day. You go home, rest,eat and partially recover.
This next day (tuesday) you wake up and your energy is at 75%.
You can go train and walk out of the gym at an energy level of 25%
or rest and recover to maybe 80 - 90% by Wed.
Say you go Wed after training Mon and Tue and your energy level is @ 50%
and you train your chest to failure then you leave the gym with a critically
low energy level and then the next day wake up feeling run down and tired as your body can expend the energy faster than it can assimilate the food to replace it. Just like you can run a horse until it expends all of it's energy and dies you can run yourself into the ground as far as your energy stores.
So you've trained every body part to failure 3 days in a row and your energy level is too low so you are more vunerable to bacteria and viruses and if you do get a sore throat or a cold, you have to fight of the germ invasion and take away from re-building muscle. So you are overtraining and not making the same gains you could have if you allowed yourself more time in between workouts to recover (recover your energy levels not just re-build the muscle trained and keep your system healthy with an ample energy store).
Now when I say you, I am really talking about myself and I'm not knocking anyone else's routine. I have oversimplified the whole thing and I could usually go a couple of weeks before hitting a critical low in energy level.
Also, the more you train the better your body adapts and supplements of all kind help you recover (even if your just taking some b and c vitamins).
As you get bigger, your body has more capacity to store energy.