Having a good marketing plan is very important; it provides you with some information that enables you to measure how you are progressing. This can then highlight strategies that are working for you and those that are not. It can also help you to determine if there are any gaps in the market that will provide new opportunities for you. Your marketing plan will also help you to focus on your target market.
Moreover, an excellent marketing plan will also benefit you in that it provides your outside investors with confidence that you know your market and that you know how to achieve your objectives.
However, having a marketing plan is not enough; it should also be effective. Tara Gentile published an interesting article at YFS Magazine that can help you determine if your marketing plan is good enough or it needs to be changed.
You can find the full article here: http://yfsentrepreneur.com/2013/08/06/is-marketing-the-problem