Who should see your plan? All the players in the company. Firms typically keep their marketing plans very, very private for one of two very different reasons: Either they're too skimpy and management would be embarrassed to have them see the light of day, or they're solid and packed with information . . . which would make them extremely valuable to the competition.
Captured thinking: You don't allow your financial people to keep their numbers in their heads. Financial reports are the lifeblood of the numbers side of any business, no matter what size. It should be no different with marketing. Your written document lays out your game plan. If people leave, if new people arrive, if memories falter, if events bring pressure to alter the givens, the information in the written marketing plan stays intact to remind you of what you'd agreed on.