That's a harsh generalization. My prep coach (who shall remain nameless, even though he does post here) changed my diet DAILY during the last 3 weeks of my prep. 0 cardio and the least amount of gear i've ever ran. You need to find a coach that works for you, before finding my coach I did my own prep's with great success. However in the final weeks I would second guess myself frequently and it led to me stringing myself out more than once. Having an experienced coach that you work well with is an amazing thing. If the trust is there it allows you to become machine like with your focus. I don't advocate prep coaches, I advocate finding the "right" coach for you.