Radiosity is viewpoint independent, which increases the calculations involved, but makes them useful for all viewpoints.
The technique works with diffuse surfaces only. This way, form factors can be precalculated and are independent of the viewer. The lighting needs to be updated only if some light source changed. If, on the other hand, the technique would support specular reflection, the form factors would depend on the viewer. Form factors and lighting would need to be updated constantly when the camera moves.
How does the limitation to diffuse surfaces increase calculations? Diffuse surfaces need to take light from all directions into account, which is more complex than taking light only from a smaller specular lobe. Is this what this sentence means? Is it just me or should this be rephrased?