If this were implemented, they might as well link all the cities. If there were actual physical wards with retainers, then, yes, not linking the cities makes sense (in a sense). If we're just going to be summoning the retainers, though, might as well let them be summoned to any of the cities, the same way we can summon our own retainers.
I agree, for reasons I mentioned in the other thread linked in the OP, at this moment I am in favor of linking the market wards. Although the developers may have other plans that justify not linking them, so I just wanted to point out this system would be flexible in both and additionally would be a much more sensible way of linking the market wards if they chose to do so.

I am slightly concerned, though, about what would happen with more than one person trying to summon the same retainer. I'm sure they have something to keep different people from buying the same item simultaneously, but I don't believe they've had to consider different people trying to summon the same retainer. This is pretty much the only hitch I can think of off the top of my head.
I did consider this problem, I think there are a few way to work around it though. Either simply allow multiple players to summon a single retainer and have the items update in real time so a player would figure out quick if someone else is buying from the same retainer. Or if that is impossible, items could be purchased directly from the NPC @ the Item search counter.

As something of an aside, they would certainly need to change some other features slightly, especially if they did link the wards. Things like being able to see more than just the cheapest 20 of an item, changing tax rates, etc. Not an issue, per se, just something I felt like pointing out.
I agree with this point as well, hopefully they have teams working on making the current system as friendly as possible while they work simultaeneously on an overhaul.