I think the main advantage of a center channel speaker is that it allows you to easily adjust the volume of only the dialog while leaving the overall volume the same.
With most or all 5-channel digital mixes, the center channel has the dialog and little else. Despite my efforts to properly balance my system, some movies have the center channel sound a little too quiet or, less often, too loud. If you have a separate center channel speaker, it is easy to adjust that just for one DVD so people don't have to strain to hear the dialog, no matter where they sit. If you rely on a phantom center channel, its difficult to adjust the volume of the dialog alone.