I think the strongest Product Designers wind up taking on responsibilities that PM's might traditionally hold (and vice versa).

I'm fairly biased here, but I'd argue that product vision is often best executed in the hands of product designers rather than PMs. Having a design leader who takes the product or feature from high level goals, to flows & interactions, and finally down to the pixels often leads to a less-compromised end result. I usually find that singular vision beats design-by-committee.

Of course, this rarely happens due to company politics, which PMs tend to excel at versus designers.