This article at Management Issues brings home the fact that poor succession planning can really hurt a company. According to the article, when a top executive leaves a public company, stock prices tend to fall. But when the company has no succession plan in place, they fare much worse in terms of stock price than those companies that do.

From the article: 

It found that companies with clear succession plans performed more than seven per cent better on the markets a week after their change than those that delayed appointing a replacement… Companies with unplanned successions – where no replacement was immediately announced to the markets – saw their share prices fall by 2.1 per cent more than their peers with planned succession processes.