My answer to the question of why to build a Sphere or Ring world? -- Because I can.
I am a newbie (more or less) and have only played single player games vs AI opponents. In all of my games I made a Ring or Sphere world...just because I could and it looks cool.
The production boost you get from such a huge world is very nice, though I already have the game mostly won by that point -- it is only a matter of time.
Cheers!
Trajan