The advantages and disadvantages of a narrow span of management are clearly set forth in the previous responses. What has not been discussed thus far is the importance of context. In some industries and in some settings, the advantages and/or disadvantages are magnified significantly, and when structuring an organization, those advantages and disadvantages must be weighed carefully within that industry and context. For example, if one is setting up a manufacturing assembly line, are the advantages of closer relationships between supervisor and employees worth the cost of a small span of control, when there is little justification for a small span of control in an assembly line? On the other hand, in a research and development department, where a supervisor is expected to foster creativity and promote employee growth, the cost of having a small span of control might pay off in the form of new and lucrative ideas.
It should also be noted that while a small span of control does tend to make for a "tall" organizational structure, it is possible to design a flatter structure, whereby some of the higher levels have a somewhat greater span of control. In some industries, the most important span of control is at the bottom, and one can rely on good middle and upper-management to competently handle greater spans of control. An example of this would be customer service organizations, such as call centers. The availability of the first-line supervisor tends to have more importance than in other settings, while mid-level managers are mostly number crunchers. If there is a problem of any sort with an employee, it is that first-level supervisor who will be implementing a solution. This requires a smaller span of control.