I can imagine an agent telling an author to get an outside editor (not a specific editor!) to copyedit/proofread a ms., if it has a lot of tiny errors. But developmental editing? An agent who thinks there’s “something there” but doesn’t know how to bring it out should be passing on the book, IMO, so that an agent who knows how to fulfill the book’s potential can rep it. That’s generally what happens, in my experience. An agent who sees strong potential usually also has definite ideas about how to bring out that potential and will at least offer an R&R. A hired developmental editor might end up having completely different ideas about improving the book, leading to a product the agent doesn’t want after all.