I havnt read the culture novels so im not too sure about this but here goes. I got some background from the site below, by banks himself (apparently)
culture orbitals and stuff
A bit of basic research shows that it would have to be as strong as a ringworld. I like the idea of building it from pairs of plates and cables(sound familiar?), you could apply that to ringworlds too, to a degree. An orbital would be useable if unfinished, you would just have to have temporary walls around the edge. Ringworlds could be built from orbiting plates, but couldnt be spun at full speed untill completed.
Each orbital holds 20 earths surface and 1 earth can be made into 1500 orbitals. By contrast a ringworld is made from 250 earths and has 3 million times the surface area. So by orbitals 1 earth => 30000 earths and by ringworld 1 earth => 12000 earths. So orbitals are more efficient (in terms of surface area/mass ratio) and easier to make (cos smaller).
I have found one reaseon for contruction, going back to the original dyson sphere. Since all known energy is derived from the sun (eventually) to achieve maximum efficency you would have to cover all the sun with solar panels to harness it to maximum capacity. This is a sphereworld (without being lived on). It wouldnt have to be very large, only to about the orbit of mercury at most. Surely it would be easier to start a small star separatly though? cold fusion, for example