I have 4.1 Klipsch on my desk driven by Audigy, I get full surround, sound comes from different directions and also get simulated center speaker. In other words, I get left/right and at the same time, get center in addition to the rear left/right. I am not missing any thing.
OTOH, in the living room, I have full system, 7.1 but then again, I am not setting 1.5 feet away from the screen like on my desk. In car environment, I would not care for a 5th speaker, as driver, you are setting on the left (or right), but not setting in the center so it would not make sense. Bottom line, it is not worth the $$$ or effort.