Human hands are part and parcel of how we became human and what it means
to be human. They are instruments of incredible sensitivity and breadth
of input to us, as well instruments of our output and expression. Furthermore,
there is a
tight feedback loop between the use of hands as instruments to inform us of the world and of our shaping the world. It is no accident that Michealangelo's Cistine Chapel depicts the creation as the touching of hands.
(Insert picture of creation here from the ceiling of the Cistine Chapel)
Allow for expressive media. The hands are an excellent way to communicate expressively, not just categorically.
Maximize output rate. Even as long ago as the 60's, information processing studies showed that much more information could be "output" using ten fingers (even when used in binary fashion) than with one finger.
Avoid injury (such as Repetitive Stress Injury). Current interface approaches not only don't make use of the hands, they often misuse the hands to the point where some people become partially disabled.
Special Tools for Special Purposes. Many uses, as well as many human capabilities, require special tools. No professional astronomer or molecular biologists would use the same "general purpose lens system" that could serve the needs of both. Nor would we want to buy a "peripheral limb coverall" designed to serve as either shoe or glove. Yet, we often attempt to force the human being who uses a computer to use the same input device regardless of the task or how it needs to be supported with the human body.
All of the above devices are designed to be used for identifying a specific item. Even to extend functionality as trivially as to choose two items, typically requires two separate actions. Hands are capable of producing many types of actions and in feeling many subtle variations of input to them. Each task is best served by a special device. In addition, most devices are used in particular physical contexts. Thus, the multiplicity of devices only has a minor negative impact on having the right tool easily to hand.
Each human hand is capable of moving in location in three dimensions over a range of several feet. In terms of relative discrimination, this allows each hand to be in one of thousands of locations. Even in terms of absolute location, this probably allows well over 100 positions. In addition, each hand is capable of three rotational dimensions with at least 60 distinctive relative positions of rotation around the forearm, 50 distinct relative positions up and down and 30 side to side. Of course, all this is in addition to the variety of finger and thumb placements.
Furthermore, people are capable of playing a piano more or less loudly or softly (standard musical notation specifies seven levels) and with various "attack" profiles. Notes may be played legato or allegreto or staccato. Seen from the standpoint of the potential output of human hands, current input devices for computers are pitifully crude. The skill of a heart surgeon, the caress of a lover, the snap of a home run hitter, the bowing action of a violynist, the moves of a sculptor, the brushstrokes of a painter, the waves of Hawai'in hula dancer, the speed of a sign language interpretter -- all these remind us of the incredible versatility, dynamic range, and subtlety of human hands.
In addition to the incredible versatility of the human hand as an effector in the world, it is also an incredibly sensitive sensor of the world as shown by the work of craftsmen, safecrackers, and Braille readers.
In fact, most hand tools have three parts: a part fitted to the hand (the handle), a part that connects with the domain, and a translation device between the two. In golf, e.g., there is a handle (slightly different for a putter, a wood, and an iron), the surface that hits the ball, and the shaft that transmits energy between the two (also slightly different among clubs). One could argue that the computer is so flexible that a single handle could be made to fit the hand and all the requisite translation and application interface could be done "in" the computer. In theory, this might be so, but in practice, the general purpose "handles" that actually exist are woefully inadequate. In video arcade games such as virtual golf and motocycle racing, the designers wisely give people completely different "handles."
(Rough sketch of handle, shaft, and tool-face)
The way in which the "accommodation" to different uses occurs may change over time with changes in technology. One can imagine, e.g., a future "smart" golf club driver face that changes properties as it "senses" whether you are hitting with a closed or open face; a sand wedge that changes with the dryness of the sand you're hitting through. Currently, however, we have erred far to much on the side of "one size fits all" (all hands as well as all purposes!).
Although hand tools may require preparations that take time, once the
craftsman begins using a hand tool (or related set of hand tools), it is
easy to take up or put down. Typically too, the look and the physical
feel of the hand tool, as well as the instantaneous effect on the material,
give immediate feedback that confirms that the right tool has been chosen
as well as reminding the user how to use which tool. Each golf club,
paint brush, and surgical tool feel, as well as look, slightly different.
McCollough, Malcom, Abstracting Craft. Cambridge, MA: MIT Press, 1996.