We know that humans act roughly as if they have a limited supply of willpower. That is, it is a reasonably functional model for predicting a variety of experimental outcomes. As a rule of thumb for purposes of providing self-help advice that applies in most cases, this seems pretty sufficient.

The problem is that there is some evidence that indicates believing that willpower is in limited makes it a limited resource. http://www.nytimes.com/2011/11/27/opinion/sunday/willpower-its-in-your-head.html?_r=0

Holding a belief that reduces your willpower in turn isn't good for self-help purposes.