I think, judging from the 'Day-Long' seats from America, it's a question of spreading the pressure as far and wide as possible,
whilst still keeping the bike rideable. If you make a seat 'saddle-like' in shape you decrease the pressure all over.
That's possibly why Sargent seats are comfy for some - they're slight dished.
On the other hand, the saddle-like shape then puts unexpected pressure on the thighs - which isn't going to suit some riders. And you're not going to be shifting your bum off the seat for corners if you're stuck in a saddle shape.
So, in summary, wot Brian said.