Shoulder seams are a special case, as they support the full weight of the sweater when worn, and it's very important they keep their shape. I would imagine that is why the shoulders were reinforced.

I think that unless this blanket is likely to be drug around by toddlers and used for building forts you are probably just fine. Maybe take a little extra time on the cashmere pieces to make sure they are secure and stable, and do some research on seaming and have a few options before you get started.