I'm looking at the ingredient list for Alton Brown's aged eggnog recipe, and I see the following:  1 pint half-n-half 1 pint whole milk 1 pint heavy cream   Does the half-n-half play any specific role here?    I calculated the final fat ratio of the mix and it's ~20%  Wouldn't it be easier to just mix milk and cream to get the right ratio, skipping the half-n-half altogether?  By my math that's 3.25 cups of milk + 2.75 cups of cream. 
The main purpose of the one part of Half-and-Half is to aid in homogenization of the one part of milk and one part heavy cream.  Per this article (toward the end):     Half and half is also a good solution for desserts that call for equal parts milk and cream, but have a tendency to separate. The fat has been homogenized in the half-and-half, so using it in desserts like panna cotta will deliver a smoother, more consistent result.    The half-and-half also enhances desired texture/mouth feel properties. 