To make FEW clams one doesn't need many W2 mages. There are some Astral spells that are nice and require gem, but until the late game they mainly consist of Astral Healing and Antimagic, and maybe Power of the Spheres. And you don't need many mages casting those... Well, Arcos and Pythium could use PotS as their mages have many kinds of magic, but making clams is easy for both.
I also think that for the Astral nations you gave Arcos certainly is able to easily make clams, or atleast has a good change for it. Even without Constr.6 they will probably have W2 mage without anything better to do quite soon. Mystics are great!
This leaves Ermor and Marignon as strong astral nations without clams. The strong astral nations *with* ability to do clams are Pythium, Arcos and Atlantis. The weak/non astral nations that can do clams are Caelum, Jotunheim and Tien Chi. 3 Astral nations that can do clams (easily) and three non-astral nations that can do them easily doesn't seem to be in favour of the non-astral ones.
Also, I think that clams were just added as a cool item without thinking how, exactly, they would be useful, and for whom. Maybe one of the developers was annoyed with micromanaging gems for his mages.