I tried this once way back in the Dolby Pro Logic days. I had a Yamaha DSR-100Pro processor and 2 pair of the Klipsch Kg4s. I didn't have a center channel yet, so tried the Phantom setup. It sounded OK, but DPR isn't very critical.

I can guess a phantom center would sound fine as long as you have good imaging mains. I've always been impressed with all the mains I've used, especially the Klipsch Kg4s. They image so well it hard to tell when a center is being used.

I agree it's definitely possible the Phantom center could sound better than using a center speaker. Moreso if the center isn't matched, or up to the task. I would much rather have a center to provide better control over the dialogue.