A good template will provided a step_by_step approach with explanations as to why each step is proposed. The strength of the marketing plan derives from the integration of each step. So if you leave out a step, you may undermine your own plan.
You can also do that if you are rushing to get a plan created. This will make your job a lot lighter because with a template, you would just be filling out a form. No need to think that hard and you can also make sure that its content is complete. Since the person who made the plan previously have revised it several times, it is sure to be accurate.