What?! You haven't noticed that the nature of time generally prevents us from doing two things at the same time or being in two places at once? :-)   That might be one reason we focus so that they can be ordered, multi-tasking is possible but degrades performance in each individual task and can be totally destructive, for instance driving a car whilst talking on a mobile phone. Overall it has a benefit, being purely linear means things would move very slowly indeed. There are many who take advantage of the cons of divisible attention, it is done during chess games, during warfare, psychological manipulation, getting a girl, whatever your heart may desire. A mistake is made and one jumps in.

Were you think about something else when you wrote this? Using "weather" in place of "whether" illustrates an inefficiency I already covered and suggests you might be under the weather.  :-)

great question.
I think it's not unlike that fact you hear that "we only use 10% of our brain." I always thought that was a bit odd, like saying that only 10% of the volume of a living room is taken up by furniture and people at any given point. Sure, but the space left over is important! Just because it's not obviously used doesn't mean it's not critical. Who wants to be in a living room without a lot of space to breathe and large windows and lots of light streaming in?

So multitasking seems to me somewhat like moving the furniture from your friend's house into your living room temporarily for storage while s/he has some painting done. For a little bit it's no big inconvenience to trip over another sofa or dresser because even though it looks a bit cluttered, there is no immediate issue (you can still fit into the room to sit on your sofa). If it persists for too long however you will likely start to feel less satisfied with sitting and reading in your living room, and this dissatisfaction may be pointed or vague. As a start, the whole experience will have less aesthetic value and probably be less conducive to creativity -- as we know creativity needs high ceilings, empty space and empty time of all sorts, and most importantly silence. (This isn't to say no music, etc but rather to say a certain singleminded focus on the task at hand.)

As long as you make room in your life for deep uninterrupted things, multitasking occasionally may just make life more interesting. Many of us, though, multitask by constantly doing things which are more than a little addictive, like checking email on cell phones or looking at websites while on the phone. The more these things become a habit the more, I think, one is likely to choose things to do which allow for e.g. email to be checked at the same time, and for the habit to be perpetuated. And thus less likely to choose to do whole genres of interesting things which require isolation, space, uninterrupted dreaming.