What is the purpose of creaming butter with sugar in cookie recipes?
Creaming puts the air bubbles into the mixture. The baking powder only helps enlarge the bubbles, not make them.  In cookies the creaming plays another essential role, which is to help dissolve the sugar.  To cream the butter keep it cool and do it for a few minutes (at 65&deg;F, harder in the summer).

It has recently been discovered that cookie dough is different from cake batters. Sugar is part of the structure of the cookie and not just a sweetener, tenderizer, and browning agent.  It forms the base upon which the fats and the starch granules of the flour are embedded.  The sugar needs to dissolve for the matrix to form.

If you beat the whole thing, it will be harder to get bubbles in and you may end up overworking the dough.  The cookies will end up flat and tough.