There is truth to the concept that secondary movers like biceps on back work and triceps/delts on chest get work 2 x a week on once a week splits. If you do combo splits like push/pull/legs, you won't get the intensity on the secondary muscles you would if you trained them alone in a one workout a week split.. As a result, you can run that kind of program with a little less recovery time. This is something we all have to work out for ourselves as we learn what our bodies respond to. Make no mistake, though. You have to recover properly to gain.
I completely agree with MnB, and I would also like to add that when training legs your back sees quite a bit of work when you're squatting and the same can be said with regards to deads or heavy rows as they will tax the legs as well. Also the same can be said with regards to all upper body pushing exercises as these IE bench, inclines, dips will affect delts as well as triceps, and shoulder pressing (all types) also tax the pecs and triceps so when one looks at training from this perspective one can see that certain body-parts will receive large volumes of collateral work through-out a training week.
And then one must also consider the CNS (central nervous system) in the whole recuperation senario as well, or recuperation will take a major set back.
