I've long reckoned that multitasking performance is similar to committee effectiveness. In the latter, effectiveness is inversely related to the square of the number of participants; in the former, performance (or effective intelligence if you will) on any single task is inversely related to the multitasking level.
When I hire analysts and other technical people, I want them to concentrate on the task - note the singular - at hand. If I interrupt with another task, I very much like to be told that I'm interfering with productivity. Sometimes I need to do that, but we all need to see clearly the cost of doing so.
[PH because she calls for undivided attention]