Hi, I think you need to distinguish between Google Desktop and the Google Search Appliance in this context. Google Desktop is aimed at indexing your personal information for your needs, the role of the Google Search Appliance is to create a single index of Enterprise content that can be searched by users across your organisation.
There is specific connectivity between the GSA and Documentum that respects all of the Documentum security setup and provides great relevancy, something which is not the case with the inbuild search. You also have the ability to add other content into the index from other sources such as Alfresco, file servers, webservers etc to provide true enterprise search.
Federating search queries will kill the relevancy and is not recommended. For great serch quality you need to get all of the data in a single index.
Documentum is a good Document Management System but if the requirement is for good search then Documentum dont have their own search engine. they embed a third party solution in their product but it wont help if you need to search across other repositories.
I hope this is useful.
