In "Why Reference and Instruction Librarians Hate Federated Searching", the author points out a bunch of shortcomings of these tools, but mid-article reveals the reason:
Given the above, why are federated search products still on the market, and why are libraries still contracting with vendors? What has changed my own mind in the last five years, transforming me from a reluctant community college reference librarian fighting it tooth and nail to a web librarian petitioning the university’s budget priorities committee for extra funds to pay for it? Two things: usability testing and developments in federated search products themselves.
It goes on to mention new features, new attitudes, and the real-world impact of simply exposing all this quality information we have tucked away in our many, many subscription databases: trying to simplify our present too many interfaces, paths, requirements, dead ends, and opportunities for confusion.
Oh! Also, Don't forget to check out http://federatedsearchblog.com/