I'm using quite a high-end four speaker arrangement at the moment, with no room correction and frankly the sound is pretty amazing (fuller, richer and more 'scary' good than with just the two channels).
Speakeers in front are Audio Physic Avanti III, rears are German Physiks HRS-120; I have them at the rear because they are omni-directional and so are easier to place and seem better able to produce that ambience. I'm likely selling the German Physiks as I had felt I only need one pair of speakers and they seem a bit of an extravagence having speakers costing over $30k for rears ;-) But, I'm loving the experience. When I do a/b comparison, it's just so much better with the four speakers. I think the key is having omni directional at the rear which fit in and disappear much better, even without room correction. My 2-cents; maybe it's not supposed to sound better, but believe me, it does.