In Dom2 clams featured a dominant role in many games on large maps. I remember a game when the limiting factor was the size of the magic item treasury in terms of clams/turn.
I think that the changes in Dom3 were for the better and they are close to balanced now. I have yet to see a game where they are decisive.
Heck, there are very few nations with mages which can forge them. Oceania and Jot are the only two that come to mind.