Using templates certainly speeds things up and it makes perfect sense to get hold of a template if you don't have the time to generate a plan from scratch. The benefits of using a template are huge because you can have a lot of information you never even thought of already sitting in the template ready to use or customize to suit your particular project.
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. Some of the better marketing plan templates provide detailed instructions for newbies and do not presume that they know how to take each of the steps required in the template.