From The Atlantic. http://www.theatlantic.com/doc/200906/steve-jobs
The short answer appears to be "having a leader" matters, but since most leaders are relatively close in ability and since most times and most organizations don't really have great need for strong, clear leadership, "who" that leader is, usually doesn't matter all that much . . . unless you're in a crisis period, working for a closed & heirarchical organization, and in a field that typically sees lots of dramatic change. In short, Steve Jobs probably matters, but most CEOs don't have that much of an effect on the performance of an organization . . . on the positive side, that is. One bad leader, especially in an organization heavily structured around the figure of the CEO, can do incredible damage. Better to have a system that produces a string of "mediocraties" and doesn't rely overly much of the leader but on the strengths of entire organization than to have one great leader and come to rely too much of being able to replicate that great leader over and over again. All it takes is one bad apple in a leader-driven culture to destroy the work of many, many good leaders.
Business scholars have spent more than half a century of research to come to these conclusions. I think political scientists and historians could have told them something similar a few millenia ago (read the Greeks) or for sure a few centuries ago (read Hobbes, Locke, Montesquieu, etc.) about the nature of leadership in large organizations, but other than Sample and Drucker, how many leadership gurus have read the classics? Hmm . . . (spoken as an historian, obviously). :-)