If current player behavior is anything to go by, this will not have a significant impact. People who accept Grid and Limsa local leves now do one of the following:
1. Do the locals as fast as possible, then go back to Uldah
2. Pick up the locals, go back to Uldah, do the synths, drop off locals later.
Even after the patch, there still won't be any compelling reason to stick around. There will be more of a reason than now, yes, but will it be enough? I'm not so sure.
There's still not enough total people per server to make all 3 wards in all 3 cities viable yet. Even with airship transport, it would still be in everyone's best interest to just consolidate their retainers in one city. The game might as well be designed around that concept.
We need a mailbox, but I don't think that's the only reason why a unified ward would work. I'm not sure what your reasoning is here.
I thought this part was obvious. You enter ward from Grid, you exit to Grid. You enter ward from Uldah, you exit to Uldah.
In effect, this actually acts sort of like a makeshift mailbox, since you can always meet someone to trade, even if they're in a different city. Just meet in the wards.
