Why is having at least some vacancy considered to be optimal?

	I.	Having some vacant space reduces your OpEx considerably.
	II.	Buildings are difficult to design in such a way that all the space will be used efficiently. It is thus inevitable that some vacant space will remain.
	III.	Both parties on the demand and supply side of the market will maximize their profit by searching for better deals. This search time results in space being held vacant, waiting for better deals to show up.
	IV.	Vacant space allows you to more easily redevelop the property.