Hi Adam,
Thanks for your input!
No, you wouldn't need to maintain anything in ProductWidgets, it automatically connects to your product database in Shopify and use those products for your widgets. When you add or delete products, or change a price or product description, ProductWidgets notices this immediately and this will be reflected instantly in your widgets. No need to worry about duplication or products being out of sync.
I like your idea about displaying products from a service like Society6.com on your shop! Technically it would already be possible with ProductWidgets. I will put it on my list of features and ideas! :)
For the differences between ProductWidgets and Shopify Widgets see my previous answer.