I think the key is consistency, and it is consistency that is hard for a company to guarantee. If a company does not promote often, like Apple, employees will be content to stay where they are. But once the company starts to promote some people in a short time frame and especially some think the promoted do not deserve the promotion, the employees will start to envy, to demand a speedy promotion, and the company will slowly, sometimes even quickly, turn into a promotion-oriented culture. Case in point, working in Google was such a prestige 15 years ago, and few people would even think about reaching E6. What about now?