Interesting points Alan, though the question remains why introduce comb filtering artifacts unnecessarily when care should be taken to reducing comb filtering through treatments or judicious speaker placement. The main problem with dual identical centre speakers is the havoc they play with dialogue because of the cancellations and reinforcement at various frequencies right across the room not just on axis.
Audibility of comb filtering depends on several factors including frequency, delay, phase, reflection etc. While in some rooms the two speakers may act to anchor sound in the middle of the screen, in others there could be directivity to one or the other speaker because of the comb filtering. Duals will not always centre the channel in the middle of the screen because of these artifacts. The ear will tend to localize the sound to the nearest loud source particularly HF sounds.
What I've always liked about dual centres is the extra spaciousness from the centre channel which can be pleasing. It seems to me that whether one gets an overall sonic improvment depends on seating arrangement and room acoustics and how much you want to trade off dialogue clarity for a more "airy" sound from the centre channel.
I've found a well designed, properly placed single horizontal works better in most rooms. Though that configuration also has issues depending on the quality of the speaker, which make it less than favourable than a single speaker on plane with identical mains.
