I like the idea overall, though I do have a few comments.

1. 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.

2. 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.

3. 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.

Anyway, like I said, seems like a good idea, overall. At this point, I'm just hoping they do something to fix it.