I believe that there is an incorrect mixing of terms here. A "Sound Bar" is not a phantom center channel, but an actual speaker that tries to produce the sound similar to 5 discrete speakers. I have never listened to one so I cannot comment how successful this is.
A "phantom" speaker (center or any other) is a missing speaker so the AVR has to reroute the audio that is dedicated to that speaker to other "real" speakers. Many people (based on comments that I have read here and other places) seem to prefer a phantom center channel vs. a full 5 or 7 channel speaker array. They believe that most center channel speakers are inferior to their main L & R speakers, and therefore degrade the total sound presentation.