You raise an interesting topic. I think the answer depends largely on the size of the organization and the size of the person. Certain large companies like Apple and Steve Jobs and Virgin and Mr. Branson come to mind. Those companies and leaders come to mind as a pair when we think of them. They are large personalities who have had a tremendous impact on the organization. Then there are companies like Coca-Cola and BP Energy where maybe the leader has an impact but not an overarching impact. On the other side, in private companies, the leader has a direct impact on the culture.
This particular link in person and culture shows the necessity for further investigation and potentially due diligence in order to properly plan and execute a meaningful integration plan.