The S shape of hammers might be a carry-over from the friction flintlock hammer, with its circular arc to scrape flint against steel; the percussion hammer appears to have been designed to hit an angled nipple square on with the most force; the pinfire hammer had to perform an awkward arc to drive a pin downward; and the centrefire hammer was more of a return to the angled hit of the percussion nipple, with a striker instead. Noseless hammers could hit a striker more in line with the barrel which, while sufficient for the task, might not deliver as hard a blow as the slightly longer arc of the angled striker? A physicist and mathematician might provide a better answer.
One thing, the tighter S shape, angled thumb pieces, and lower positioning of the tumbler vis-à-vis the line of the barrel on later hammerguns means that when the hammers are fully cocked, they are out of the line of sight. The other extreme are pinfire hammers which, when cocked, offer a sight picture resembling rugby goal posts!
One thing that is remarkable on so many of the centrefire hammers pictured above is the retention, though highly stylized, of the percussion-era 'cap guards' on the hammer noses designed to keep flying bits of copper cap away from the shooter, a good example of skeuomorphism.
As posts without pictures are dull, here are Lancaster early centrefire hammers of the noseless variety, dated 1858.