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.
The template itself provides guidance and milestones so that you can measure your progress. If you are not achieving the results you want, you can revisit the template to check that you have not overlooked something or misinterpreted what is required.