There are actually two reasons why we should reduce all our tasks, goals and activities to a small number of the most important ones.
Actually time management experts write about it, though usually not stressing it explicitly. For example, the title of a Stephen Covey bestseller is "The Seven Habits of Highly Effective People" (the highlight of word "Seven" in the title is mine). Covey also recommends preserving and enhancing four greatest assets you have (Physical, Mental, Social/Emotional and Spiritual).
Drucker believes that there are few people, who can effectively manage three important tasks simultaneously. We notice here that managing tasks or solving problems is more difficult than just keeping them in mind. Three is the maximum amount of variables, relation between which most individuals can understand. If we make the rounding, 3 is the number of the "bits" (tasks) that via the rehearsal allows storing 23 - 1 = 7 "things" (we subtracted one state with zero information).
We may look at it from another angle. If we recall that 7 is the limit, we understand that only the trained and experienced people like managers usually can easily keep in mind up to 7 activities simultaneously. Most other people can easily keep in mind one half of their limit i.e. approximately 3 things or activities. Indirect confirmation of this is that three is common when a group of people wish to perform an action in synchrony ("Now, on the count of three..."). Let us mention here that time management computer program Activities and Priorities has 3 activities as the starting preset that is enough for most beginners who until they are trained in any case find it difficult to manage more.
And finally, Hillary Rettig makes a similar point in her article for Microsoft "ChannelBiz". She writes about a to-do list but it is clear that the same is true for any set of things that we would like to keep in mind simultaneously: "As the number of items on your to-do list grows, your ability to accomplish your goals diminishes. (Having a long to-do list will also, ironically, make the task of time management itself take longer.) Your goal should be to come up with a daily list of fewer than ten - or, better yet, five important tasks".
"The key is determining whether an activity truly offers value." (Peter Drucker)
"Identify and eliminate the things that need not be done at all" (Peter Drucker)
"Doing the right thing is more important than doing things right" (Peter Drucker).
"Things which matter most must never be at the mercy of things which matter least." (Johann Wolfgang von Goethe)
"If you want to make good use of your time, you've got to know what's most important and then give it all you've got."(Lee Iacocca)
"You have to decide what your highest priorities are and have the courage--pleasantly, smilingly, nonapologetically- to say "no" to other things." (Stephen Covey)
"Focus on a Limited Number of Important Tasks and then Ruthlessly Eliminate Everything Else." (Hillary Rettig)
"How different our lives are when we really know what is deeply important to us, and keeping that picture in mind, we manage ourselves each day to be and to do what really matters most" (Stephen Covey).