when I do the pullup portion i alternate my grips palms out (pullup) palms in (chinup) palms faceing each other. I do this because I am still week in the pullup department and can do more in less time this way.
That being said, I am going to postulate that this IS a good practice. Changing the grip changes the muscles that are affected. Therefore, changing the grip allows you to more fully work more muscles. I remind myself i am doing this for fitness, not just to increase my number of pullups. It also allows us to work the large back muscles more since when they get fatigued we can change grip so that the arms carry a bigger portion of the load. Changing grips also allows us to get a better arm wrokout because so much more load is carried by the arms during a chinup. Lastly, this enables us to do more volume resulting in a bigger impact on the nervous and cardio vascular system
those are my thoughts anyway.