"Ask the experienced, not the learned." - Arab quotation
Actually I didn't phrase the why question very well. What I meant was why PRACTICE on hard surfaces. From experience, as you improve your rolls and they become more automatic, you will find you do them in just the situation you mentioned, but you can avoid the possible complications of practicing them on a hard surface can cause. If you can roll, if you roll alot and it becomes ingrained in you, it wont really matter where you roll. Like your friend, a few bumps but no biggy.