"Plans are useless; planning is indispensable," according to Dwight D. Eisenhower, then Commander of the Allied Forces in Europe during WWII. Now, you may be in total agreement with the first part of that statement, but you are really not convinced of the truth of the second part.
The "One Pressing Issue" Plan: Business planning does not stop the day you open for business. Under the best of circumstances you should be revisiting your plan once or twice a year to see how things are going, and where perhaps you've veered away from your original goals. Remember, changing the direction of a business isn't always bad, but it should be intentional.