What is the job of a manager of a publicly listed company?

The American magazine Businessweek recently featured an interview by their Silicon Valley correspondent with Clayton Christensen, author of a book called the Innovator’s Dilemma. The interview concerned Apple Computer’s ability to build on their recent successes, rather than hitting the crunch they had when the PC platform overtook the Mac in the early 90s.
 
It is an interesting consideration of the shift in a maturing market from the proprietary dominance of the early market to the modularisation and commoditisation of the product as the market matures.
 
The fact that really got to me is the second last paragraph. Apparently in the states, 10% of all shares are held by hedge funds that typically hold an investment for 60 days and 85% of equities are held by mutual and pension funds that typically hold onto an investment for 10 months.
 
I find that staggering, I’ve long held a cynical view that as soon as a company lists on the stock exchange and goes public that they are at the mercy of stock market moods. Cast here and there, they are only as good as their last quarter financial results. I suspect there could be a tendency in some quarters to operate and get all stressed out about the market reaction to company news, especially when their bonus is tied to the share price. But the correct approach, as pointed out by Christensen, is the longer term health of the company, not short term and therefore unhealthy practices.
 
What is the job of a manager of a publicly listed company?