The main reason I'M building a ringworld is because I can. And they're cool. A sphereworld is an even cooler idea. Why would you need a better reason? If the only reason you're playing is to beat up on a human or on a bunch of AI's, ignore ringworlds and sphereworlds.
HOWEVER. (The rest of this post may sound a little harsh, but it's not intended to be. Please just take it as educational in nature.) I want to point out that most of the research required to get to the ringworld/sphereworld tech is useful in its own right. If you want to finish the game with no ship bigger than a Light Cruiser, no base bigger than a Space Station, and no Stellar Manipulation techs (or just the create/destroy storms, create/destroy planets, warp point closers and lower-level warp point openers), feel free. If you want to take it to the extreme, don't research any of the ship size tech levels, base size tech levels, or stellar manipulation tech levels at all.
As for me, I researched most of the required stuff anyway, and getting to the ringworld components was just one more tech level in Stellar Manipulation. One that also includes the Sun Creation component. And getting the Sphereworld component also gets you the Black Hole creator/destroyer, the sun destroyer, and nebula creator/destroyers.
So, in summary, you're getting more than just a Ringworld for those 2,619,999 research points. You're also getting Frigates, Destroyers, Light Cruisers, Cruisers, Base Stations, Starbases, and a slew of Stellar Manipulation techs, some of which might even help you beat up on the AI.
