There's a pretty good article in Advertising Age about the benefits of widgets and the fact that not a lot of people are using it. AdAge says that "entire segment" will amount to around $100 million. That sounds like a chunk of money, and especially one for an industry that I consider a subset of rich Internet applications, but as the holy grail of advertising, it's not much money. And AdAge talks to people that bring up a number of issues around widgets including non-standardization, cost, and scale. All decent reasons, but I think we're just now getting to the point where the technologies are viable for widgets on a wide scale.
I consider widgets a subset of rich Internet applications mostly because of the technology behind them. Almost all widgets tend to be pretty interactive which makes platforms like Flash and Silverlight ideal for deployment. The fact that the embed model works so well with those technologies makes it easy to port them. Adobe AIR provides desktop functionality that lets you persist and create notifications. Then think about Flash and Silverlight's move into the mobile world and how easily it will be to repurpose widgets across devices.
That last part is key. There is a fairly sophisticated ecosystem building up around monetization of widgets. yourminis was one of the first companies to the space and have focused on customization and working with partners. Since being acquired by AOL they've been busy adding customers and partners. The other aspect is being able to analyze this content and provide analytics. That's where repurposing the widgets becomes more interesting. You could automatically track your widget across devices, social networks, and desktops with the same tracking back end.
There are also more complicated engaging widget platforms emerging. Sprout Builder is a great example of this. Something a bit more complex than a traditional widget, they provide a framework that can be customized with interactive content and then placed anywhere widgets can be. They provide the same kind of analytical tools so you can track where your widgets are going and even limit them by domain.
Widgets are a great way to engage users and the most powerful thing about the is that users can consume them in ways they control - desktop, web, Facebook, mobile, etc. So their value as an advertising engine is significant and with the ways the infrastructure is moving, it's going to be easier and easier to create and deploy widgets.