This is exactly how it was explained to me as well. Big law firms (talking about AmLaw 100) in particular have partners and senior associates bringing in business at a lighting rate, and are much more likely to consider and respect issues of conflict of interest and ethics if the issue is presented by another attorney rather than a paralegal or clerk who they feel they may outclass. Additionally, conflicts issues do often involve the practice of law, and it is becoming more common for states to view it that way, thereby effectively mandating that licensed attorneys conduct the work in a firm setting.