Why?
Because T206 Sweet Caporal backs from any series are just soooo common!
All joking aside, I collect different backs for my T206 set because it adds variety. I'm not what you would call the average T206 collector though because from day one I never had any desire to own an example of each back.
Do I chase backs? Yes and I care just as much about the backs as I do the fronts. In order to complete a back set (every card/same back), a collector has to chase the player on the front and the advertisement on the back equally.