I think you may be confused, better vertical directivity doesn't really have anything to do with this. Phantom center has tonality problems caused by acoustic cancellation and it also does not work very well too far off center, depending on various factors.
Coaxials don't fix any of those problems. A good center, properly positioned, will always sound better.
The only reason to ditch the center is practicality, if you can't fit a good sized one or it has to be very low vs the L/R, then it can sometimes be an OK compromise. But make no mistake, it is always compromised. As stereo itself is compromised.