My thought on this subject:
Obviously half of the sky (Boeing) does take off on center tank fuel.
To my knowledge this has never caused any accident.
But it is true that in theory there is a risk of contamination going to both engines at a critical time.
Flight Detent has explained very well why this does not apply to the 321.
I think the real reason is Airbus thinks they know better than Boeing, and must make everything better than its competitor.
That's the simple reason why, being their airplanes more sophisticated than Boeing (at least in their mind...), they excluded the remote possibility of this contamination problem with a prohibition of their own.
LEM