Speaking from a publishing perspective, it is generally agreed that widgets can act like a hook, hooking new readers into your content driven site.
Their detached/atomised nature enables them to be embedded in user profiles and places which form a large portion of users daily online habits.
A Canadian newspaper company Newspaper Direct are offering “Adgets” – widgets which are embedded within their site that enable users to engage with third party content eg. booking a table for a restaurant.
Ebay have had this type of thing available for embedding for a while. http://ebaytogo.blogspot.com/