The more you use your device's battery the faster it will wear down. They can only accept a finite number of charges so if you are essentially "wasting" one just for charging your iPhone I would not expect it to last as long for actual iPad working hours vs. one not used for this purpose.
To learn about 'Maximizing Battery Life and Lifespan', click here --> http://www.apple.com/batteries/maximizing-performance/
I would think getting one of those batteries designed for charging mobile devices would be a cheaper solution that unnecessarily wearing out your iPad battery which is likely to be expensive to replace.