In application development we often talk about the importance of consistency, in particular with respect to interfaces. In mobile application this is even more important.
This is one of the advantages of the RelativeLayout, the ability the precisely control where elements appear. In this case, we use a LinearLayout to stack the RelativeLayout beneath another LinearLayout. The RelativeLayout is then configured to anchor itself to the base of the screen.
I am currently using this technique in an application to provide a consistent look for your application. It gives you flexibility as well. You can use the RelativeLayout to add additional buttons to the bar. You could even apply additional layout controls to the RelativeLayout to add more buttons.