Any casual viewer of the BBC programme, Dragons Den will be aware of how easy it is for weaknesses or gaps to be identified. Depending upon the purpose of the plan, this may, or may not, prove to be critical. It is often easier to recognise such weaknesses and gaps, and be prepared to deal with them, either by noting them in the plan itself, or having appropriate answers available should the need arise.
In many cases business plans are very important but so much of the time it's a plan to try to convince someone else that you know what you are doing with your business like banks, investors, partners, etc. Now it's true that a well written business plan can also be a major benefit to your success as well if done right. It can guide you and keep you on track and can be the vehicle to get you were you want to be especially with so many outside forces now days that bombard you. A plan can be extremely important to your success especially when you look at the statistics that says 51% of small businesses fail sometime during their first 5 years.