By this I mean the agency should have contacted the candidate, told them about the company, assured the candidate is interested and available, gotten current and expected salary information, and done a pre-screening against the job requirements for skill match and for fit for the organization. Once the candidate has been presented, YOU own their information not the agency. Take the ball and run. Interviewing, reference checking, negotiating, closing should all be done by your on-site staff or managers. But before you dismiss them altogether understand that some agencies really do have an in with specific companies and industries as well as have very strong databases of their own.
Many agencies are very hard working, successful, and can be quite an asset.
I personally advocate a combination of on-site recruiters and agencies. If you have many internal resources and are building up or subscribing to job boards, contact lists, attendee lists, paper resumes from job fairs, or faxes no one is looking at, consider getting a sourcer, especially in lieu of hiring another recruiter. If good, they can do magic. Find one with AIRS training, if possible. They can free up your recruiter to do the real sales side of the job and take full advantage of the money you are already spending on these resources.