Running a phantom center offers the advantage of your center image matching your Left and Right speakers perfectly...since the sound is produced by those speakers. If you sit dead center and you're the only one in the room, it can work well. As John wrote, don't do this in stereo, but rather surround mode with the "center speaker" set to "none" on your receiver and it will perfectly divide the center channel material equally between left and right.
This works well under some circumstances, but has more disadvantages than advantages.
If you DON'T sit dead center, or there's more than one person in the room, the phantom effect won't be centered in the audio image. A center channel speaker locks that material dead center with your TV. Having a separate center channel also allows you to modify the output level in comparison to the other speakers... many here bump up the center to improve intelligibility, for instance. And matching the side speakers in tonality is not an issue if you are using three speakers across the front that are designed to b used together.
