I have a multi-material product that consists of two parts, in the description there's a button (variants mod) to purchase the second part. When someone pushes that button the product will be added to the shopping cart in the default material, the button is a nice feature except there's no option to change the material at checkout.. This problem will also occur when someone list several ring sizes with the "variants mod" from /labs, they would have to make buttons for different sizes and materials (for different materials they would need multiple model pages with a different default material right?) I can't see any reason why someone wouldn't like to see the material picker being reintroduced to the checkout page.
Shapeways removed this capability to change material options from the cart page because they felt it caused too much clutter. I'd say hire a new efficiency expert to re-evaluate this issue.