I want get as much useful information out of the mouse as possible, in
ways that are:

	Practical
	Intuitive
	Efficient

Pie menus are practical because they're general purpose. They can be
implemented on many types of hardware, and they can be used to input
many types of data. 

For a mode of interaction to be intuitive, there must be an obvious,
well known, or natural correspondance between the visual presentation,
the actions that the user can take, and their effects.

Efficiency depends on the ammount of attention and precision needed to
pass information from the user to the machine, and how productivly the
information is used.