I don't know why are people opting for a no or a 'i don't care' because it doesn't hurt having a thinner chasis.
A thinner chasis, if Apple designs it right, will not hurt battery life. One example of this is the Droid Razr Maxx which was had a huge battery bump (1780 to 3300 mAh) compared to the Droid Razr. The Droid Razr had a razer thin profile (like the name goes), and the Razr Maxx had a slightly thicker profile but can still be considered as very thin . (8.99mm from 7.1mm) This is all due to the high density battery they decided to put in.
Furthermore, the new iPhone is slated to have a much 'longer' screen, which means there is more space in the internals to place more parts especially the battery. They could make the battery thinner yet 'longer' to accomodate the 'longer' size of the phone and yet achieve a higher capacity to retain the iPhone's long battery life because of its size, not thickness.