Originally Posted by DrofDissonance
I know from experience that in a room with its first significant reflections dealt with (or the room is too big to matter) that having the same signal coming out of the left and right speakers makes it sound like the sound source is a phantom speaker in the middle.
This is not the case.
Indeed if you sit in the center of 2 speakers whose signals are identical, it will appear that the sound comes out of the space between them. Now, in a theater with a 25' or even 50' screen and speakers far apart, if you are sitting on the left side you will hear the sound as coming from the left side. Remember, the signal is identical but you moved so now you are closer to the left and farther from the right. One thing the center speaker does is anchor the sound in the center. Sit on the left, the sound is coming from the center. Sit on the right, same thing.
The other reason to have a LCR is for headroom. You can pack more into 3 speakers than 2. I am not talking about making loud, I am talking clean. Also, balance.