Rejection letters are not unknown to authors. Even the top best selling authors have had their share of rejections. And having a best selling title is no indication that others will follow. While published authors stand a better chance of getting new books published, there is no guarantee that this will happen. It is not surprising that a published author may not get published again. This means that successful authors are not immune to future rejections. Obviously form letters cannot possibly have any degree of redeeming value. For the most part established agents receive an inordinate amount of queries, many of which describe projects that they can't sell. Since rejections are of no benefit to the agent, any attempt to respond with specifics is not practical and results in a loss of valuable time. In short, agent rejections are for the most part handled with the short, polite rejection slip. Often the rejection is a result of a poor match. Perhaps the author did not do an adequate research of agents handling the genre in question. There are times when an agent is overloaded with incoming queries while working on current projects for submission, or negotiating several book deals and publishing contracts. These conditions often prevent the agent from considering less profitable projects.
It is generally accepted that agents will send out form rejection "slips," rather than a more formal 8 1/2 x 11 letterhead. Hence the established practice of including an SASE with the query. Nowadays, with the convenience of email, most agents accept this mode of communication and have a prepared "form" rejection delivered with the click of a mouse.
On the other hand, rejection letters from editors may have in some instances occasional redeeming value. Seasoned, highly successful editors do not send form letters - certainly not to an agent. On rare occasions I have received such a response. I then notify the editor that this is unacceptable and that I am removing his name from my submissions data base. Authors need agents, and agents and editors need each other as well as authors to keep the industry functioning. Protocol dictates that an editor needs to respond to an agent in the form of a letter, however brief. With a hard copy submission most editors who reject a work respond with a letter and a return of the material. With an email submission via attachment, it is appropriate for the editor to respond via email.
Essentially then, a rejection letter from an editor is more apt to have important clues for an author, than one from an agent. The editor sends the letter to the submitting agent who then shares it with his author/client.
Sometimes the editor rejects the work because it would compete with a forthcoming or published title and will state this. Or, there are currently several competing titles already out there. At times an editor will remark about plot, pacing, characterization, mood, etc. My advice to authors is that if they take every remark as gospel they will surely go bonkers. If the same clue appears from two or more editors, then there may be something to consider. On the other hand five different objections from five editors may not amount to a hill of beans.