Likewise, there are other professions for which the same principles should be applied. Joel names one, famous singers.  Other examples would be Lawyers, Consultants and the like.

To a degree, this applies to every profession that provides its own infrastructure.  It may not apply to the self-employed plummer (and even the plummer hires a Tax accountant).  Just getting the level of abstraction (and therefore the price tag) right, that’s the difference.