The very reason that detailed business planning sometimes fails is because it is too detailed. If you do not build any flexibility into your business plan, you are asking for trouble. The best business plans take notice of external events, and when the world changes around them the plans must be able to adapt. Of course, the opposite extreme is even worse, which is no plan at all.
Business owners without any plan tend to lurch from crisis to crisis, always breathing a sigh of relief that the last disaster was averted. However, every missed bill, delayed tax return and misuse of credit is adding up, and over time these will weaken a business. There is a way to get the best from a plan without it being overwhelming or non-existent, and that is to focus the plan on the big picture goals, with flexibility on how you achieve them.