The truth-inverting concept of 'multi-tasking' proves to be a major nuisance in modern life; in encouraging what is already a big problem of short attention span, distractability, not to focus, inability to attend, failing to be here-and-now and living in real-time.
Multi-tasking might have a reality in terms of someone who is simultaneously able to perform multiple skilled processes in parallel.
In this sense, Glenn Gould the great pianist was described as able to do more than one skilled task at the same time, each at a very high level; and this goes along with this unsurpassed ability to play the different voices in a fugue (or other polyphonic, contrapuntal form of music) as if each had independent existence.
Yet when Gould was aiming to attain the very highest level of skill - as when performing a piece for a concert or recording - he was totally wrapped-up in it; such that he seemed to be entranced and oblivious. No multitasking there!
For lesser mortals there is much greater need for unitary concentration, for focus, in performing a difficult task. And if this is lacking - then the task is being done sub-optimally.
In practice, when people claim they are multi-tasking they are simply allowing themselves to be distracted - and accepting the necessarily lower level of performance which results.
(Thus social networking while attending a lecture, or listening to loud music on headphones while studying for a test, or browsing the internet while watching TV - and so on.)
And when women claim (as they so often do!) to be better-at-multi-tasking; insofar as this claim has any meaning at all, it merely means that women are (by and large, and leaving aside pathology) worse-at-concentrating - for which there is a great deal of anecdotal as well as statistical evidence.
Highest performance entails greatest and most sustained focus: the ability to concentrate is an ability, not a deficit.
Further reflections @: http://addictedtodistraction.blogspot.co.uk