The tag doesn't do anything in my experience as well. I can't see the code does anything with the rarity tags on the element file name while choosing an element for a layer. It is only used to build metadata.
I'm using the number in the layerOrder config to induce rarity. For instance, if you want to make an element rare, create a new layer and move it under that. Then use the number in the layerOrder to make it rare.
The tag doesn't do anything in my experience as well. I can't see the code does anything with the rarity tags on the element file name while choosing an element for a layer. It is only used to build metadata.
I'm using the number in the layerOrder config to induce rarity. For instance, if you want to make an element rare, create a new layer and move it under that. Then use the number in the layerOrder to make it rare.