Their ratings are really very close. So in this specific case adding 8570m is almost meaningless. But you have to understand that notebook chassis are done for a series of processors not for a specific processor.
Chassis is then mass produced and different memory, cpu and hdd are added to achieve different notebook configurations.
So 8570 has no sense when used with mobile i5, but makes a lot of sense when used when used with mobile celeron or pentium CPU (they have much weaker integrated video).