I completely agree that AAUs are a gross way of estimating bitterness, but that's not the purpose of the AAU measurement; in a recipe they're used in place of a weight, not in place of IBU's. 4 AAU's of a particular hop added at 60 minutes will provide different IBU's than 4 AAU's added at 45 minutes, however this is true of weight at well.
What I'm looking for is a way to add, say, Centenniel hops to my recipe, but rather than specify a 1oz. addition, I want to specify a 12.8AAU addition and have it automatically fill in the weight for me based on the alpha acid content of the hops that I actually have. It removes that alpha acid variation from the equation; so that when I share the recipe with a friend, I can say "add 12.8 AAU at 60 minutes", which negates any differences in alpha acids.