I see a benefit if these are drastically cheaper, but if its just about size...
- Less battery life (smaller capacity battery)
- Less features (Apple has a tendency to withhold on smaller models)
- Less to love
Obviously the internet is filled with "Exceptions to the rule" people who will argue their need for tiny phones but in general then every (honest) person I know that whined about the bigger screens ended up pretty satisfied with the 4.7" screen. And thats because the difference in both size and feel were much more insignificant than they thought. (And pockets for a flat object aren't as tight as people claim them to be).
Maybe its for the "emerging markets" again, but in the end then I think Apple already hit the sweet spot with 4.7 and 5.5. And for me, i'd even go bigger if Apple would like to get king sized.