I can think of many situations where Google Coop can be handy. For example:
- Only add sites to the searchlist which you think are trustworthy about the subject
- Add sites which are interesting for the community of your website. You could even allow others to add sites to the searchengine. This way you can build it together.
- Create a local searchengine by only adding websites about a certain location (e.g. only sites you know about Amsterdam)
The resulting searchengine can also be added to your personalized Google Homepage.