Increased cognitive load decreases psychomotor task performance

Multi tasking though a fine idea that applies well to computers isnt meant to be all that effective in a human system. There exists a direct correlation (obviously) between the level of concentration required and the level of difficulty of the task at hand.
So when in scenarios where more than one individually simple tasks are to be performed simultaneously..then it often equals the cognitive load on the brain of a hugely complex task being performed ..and hence the concentration required greatly increases too.
Now what happens when the trade off between the increasing cognitive load and the increasing attention required fails or breaks down .. we have deterioration of the task being performed at hand ..!!
Theoretically simple .. and obvious .. but then even before Newton ..Apples had been falling off trees for millennia .
So probably it is necessary to study the relationships between the types and levels of cognitive load and their consequent effects on performance detriments.
Theory is fine .. but whats the point of it all...??
Hmmm..maybe i can answer how safe it is for a person to talk on his cell phone while driving ..!!!