One of the attributes that is spoken of as a shortcoming for id Tech 4's outdoor rendering capability is the portal
rendering model.
7318 recently made HL2 style skybox's for Vanilla Doom 3 and this was reminiscent of the old Prey Style Portals mod.
If my theory is correct, a full scene traversal in a wide open outdoor area would be more expensive than 2 or more
recursive portal sky renderings approximating the same view distance because each skybox will have a more confined
dataset which will be easier to evaluate than evaluating all scene geometry for all lights even with early outs.
Of course, BFG confines shadow evaluation to the light volume so it's less susceptible to this weakness but I image
there are still scene wide evaluations that would benefit from breaking the scene into multiple portal views composite-ed
rather than one big portal view.