Walter Kirn has a great article about the scientific results that support his claim and his own experiences with multitasking - it destroys our ability to focus.

"Multitasking messes with the brain in several ways. At the most
basic level, the mental balancing acts that it requires — the constant
switching and pivoting — energize regions of the brain that specialize
in visual processing and physical coordination and simultaneously
appear to shortchange some of the higher areas related to memory and
learning. We concentrate on the act of concentration at the expense of
whatever it is that we’re supposed to be concentrating on… studies
find that multitasking boosts the level of stress-related hormones such
as cortisol and adrenaline and wears down our systems through
biochemical friction, prematurely aging us. In the short term, the
confusion, fatigue, and chaos merely hamper our ability to focus and
analyze, but in the long term, they may cause it to atrophy."

Via The Atlantic.com