Ideally, after writing marketing plans for a few years, you can sit back and review a series of them, year after year, and check the progress of your company. Of course, sometimes this is hard to make time for (;there is that annoying real world to deal with);, but it can provide an unparalleled objective view of what you've been doing with your business life over a number of years.
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.