"thin wire - for keeping the coils thin for over lapping or for achiving the desired voltage?"
Both. The magnets are on the small side. Big wire would be hard to fit.

The center toward the axel and the outside edge run out of space for wire. It gets thick fast.  Single rotor allows the thick parts to be bent over the laminations, out of the way, like Ed did.  I run out of space in the center.

For dual rotor, someone thought about folding the 1st and last layer kind of over the magnets on the rotor.  Wouldn't be easy to do.
magnets ]I[ magnets

Either will use more 'dead' wire in the coils, to be able to get the tops and bottoms out of the way.  Notice in the link, the 1st layer of coils are a lot longer than they need to be for a 'regular' 6/8 or 12/9 configurations.

The gain seems to be that every magnet is are always acting on 2 coils and 2 phases. And active coils are always being influenced on both legs.
Arguably, a better use of magnets and wire.  I am not qualified to get into that argument.
http://www.fieldlines.com/story/2004/6/7/21190/34164