Well, I think it depends on what you are trying to achieve, what type of progress you have made working on your own, and how much further you have to go. I also think it depends on what you get when you do work with a professional, as well as what their experience/education/references are like.
In the first case, if you are making good progress working on your own, or have a handle on setting up your own nutrition and training programs then hiring a professional (at that stage in your career) may not be needed. On the other hand, if you have difficulty setting effective training programs and sticking to them, or are not making the progress you would like to see, then seeking out a professional (and researching their credentials) may be a good idea.
The same can be said depending on where you are in your training career. For someone just starting out, a decent program and nutrition can easily be found online with a little reading sweat. On the other hand, someone who is on top of their game where every small gain is a major achievement may need to enlist the help of a professional. This professional should be educated and experienced in measurement/evaluation and prescription. Understanding what type of tests need to be run to measure various aspects of fitness/physique, and then knowing how to program to increase those measures becomes very important.
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