"He is the best of the three of us at this sort of thing," the Doctor told Cayce as she glanced at Ponder.

The perils of having a mad scientist in your midst- you couldn't trust him, but if there was anything very interesting going on, you would eventually need him. If the Master hadn't been there, the Doctor could have probably been just as useful, but since they did have the Master's expertise, there was no point in risking their success on the difference.