If the agent doesn't place manuscripts with a publisher, the agent doesn't get paid. Also, agents without a solid record of placement can't get credibility. The agent will want the record, to build up credibility and business (an assessment business and an agenting business would both benefit from a solid record of placement) and will try to place a good manuscript.
If the agent has a solid record, I'd go with them. They will want to keep their average up, to increase that end of the business and to use as a selling tool for the assessment business. If they don't have any placements, I wouldn't go with them whether they assess manuscripts or not. Either they are lousy at agenting, too busy, or just a terrible editor.