Hiring internally versus externally: Which is the best way to cultivate talent?

Marketers have tended to attract the reputation of being somewhat flighty souls. It has variously been estimated that the average tenure of a marketing director is around 15 to 18 months. But this generalisation hides the fact that many marketers enjoy long tenures, working their way up through the ranks, supported by a culture of career development and a policy of internal recruitment.
The stalwarts of internal recruitment tend to be the blue-chip corporates like Procter & Gamble (P&G)