If the bazaars didn't add "extra" storage then it would not be used that way. If by putting an item in my bazaar it didn't actively remove that item from my inventory then it wouldn't provide extra storage.
If you ever played FFXI the bazaar system from that game worked exactly like that. If it was a tradable item, I could toss it in my bazaar, the item was still in my inventory I just couldn't interact with it until I removed it from my bazaar.
The problem with that approach though, and my guess on the matter, is that the new inventory system in 2.0 is not going to be menu based, it's going to be an icon grid. And rather than figure out how to make the item grid work with a personal bazaar they just scrapped the idea completely.
The server doesn't render them but it does assign a capacity for how many can fit in a given room. And it needs to support player capacity too which means that it needs servers that can support a given number of players inside one at a given time.Do you understand that the server does not render the NPCs? All it has is a bunch of XYcoordnates, and a name which links to a database containing the retainer info. The server doesn't have to do anything when there isn't anyone standing near a retainer.
One thing I like about them is it gives you the option of going to the NPC instead of buying from the menu and thereby saving you the fee if you aren't in a real hurry. This also creates an oppertunity for secondary impulse sales that doesn't exist with a regular AH system.
If we can't visit them, then it doesn't require it anymore.
As for the benefit of going to a ward to save some gil on sales tax, That's really the only valid argument for keeping them, and that can be re-balanced with their removal as well.
As I proposed earlier, allowing us to access the wards and the search feature through our retainers or through the market stall, a slight tax from using the retainer instead of visiting the stall would functionally be identical.