I am a graduate of the MBA program at NYU Stern and used to work part-time in their admissions office (but I did only administrative work and had no say in admissions decisions).
First off, I can say that few of my classmates were actually CFA charterholders, so definitely having the "CFA" letters can be a differentiating factor. Having said that, many more students did pass one or more levels without having received the charter, as was I when entering Stern.
Like others have said, you need to create a viable story that admissions officers can buy into. Thus, pursuing the CFA designation is a worthy goal but should not be the emphasis of your application essays. In fact, if you overemphasize it, then the B-school AdComs are going to wonder why you need to attend business school in the first place.
I would use CFA participation in two ways to enhance your application:
(1) Academic Capability: If your GPA and GMAT scores are below average, then passing one or more levels of the CFA exam can demonstrate your competence in certain subject areas. It is helpful to explain the specific topics that are covered in the exam as they relate to a typical first-year MBA curriculum (such as Economics, Statistics, etc.)
(2) Career Focus: This will help to show that you are dedicated to the area you are going into. However, you need to make sure you don't contradict yourself. Admissions directors are not dumb and know that going into Investment Banking or Sales and Trading does not require a CFA designation. Thus, if you are going to mention CFA program participation, you should mention that your career goals are along the lines of equity/credit research or portfolio management. This makes your story focused, credible, and most importantly answers the "Why MBA?" question directly.
Good Luck.