As mentioned above - you want to have a center channel, but it is not always practical, or even possible to put one in depending on your video setup. Most often, it is a plasma over the fireplace which really makes a center channel impossible.
In which case you are forced to run the center as a phatom. Most receivers handle this automatically when you go into setup and say that there is 'no' center channel. The system then takes the center channel info and evenly sends it to the left and right speakers.
As said, if you aren't in the sweet spot, the center channel audio will then be shifted, so it is not ideal, but definitely works when a center channel is unavailable.
As always, this is a modification based on needs of an environment and not something that is recommended in a serious theater setup. But, in a family room, it is often the only way to go until builders start to realize that the fireplace should NOT be the central focus of the room. The TV should be.