The marketing planning (concretized in the marketing plan) is an essential organizational activity, considering the hostile and complex competitive business environment. Our ability and skills to perform profitable sales are affected by hundreds of internal and external factors that interact in a difficult way to evaluate. A marketing manager must understand and build an image upon these variables and their interactions, and must take rational decisions.
When I ask entrepreneurs and small business owners, it is not unusual for not having any marketing plan. I once read that Sir Richard Branson never had any marketing plan when he started the 1st Virgin business many years ago. In fact in one study of entrepreneurship (I couldn't recall the name of the study) was that as much as 50% of entrepreneurs and business owners don't have a business plan and marketing plan when they first started the business.