Why would it matter if an East A320 was used on a West A320 route to the pilots? If the swap is a one for one, why would this even be a point to argue over?
From an "operations" viewpoint, this makes a lot of sense. And it would be no problem for crews to swap from east to west metal and vice versa.
And this would be much more efficient for the company. As an aside, however, what, then, would be managements' motivation for
EVER negotiating a joint contract?
But, there's that little matter of whipsaw. The company could do major harm to one side or the other with this empowerment. There is no way this would be done in a one-for-one fashion. It is nearly impossible to track with the amount of flying that goes on here.
And then there's this: you could bet that it wouldn't be long before the fists would be flying in the jetway of a flight where east and west crews are exchanging duties.
Yea, its that intense - with some folks.
I'm sure there are many other reasons why this would not be a viable situation. But for now, suffice to say, the TA prohibits this "interbreeding" until a joint contract is achieved.