I recently received a letter that said, among other things:
Writers deserve more feedback about their work. I think every writer deserves an honest, detailed response.
Really, why? Did you pay me to read your submission? Are you paying my electric bill, expenses, and salary? No, the people who do that are my clients, and they are the ones getting an "honest, detailed response." While I would love to give every writer a personal rejection (and I really would), even if it's to say that the work is just plain horrible, I can't. I don't have the time to sit down and read and critique every proposal I get. As it is I get angry letters and e-mails that I spend too much time on just getting a form letter in the mail. And I know I take too long responding to my clients (or at least longer than I would like).
An agent's first and primary responsibility is to her clients, not to those people who send in submissions, often unsolicited. If you want an honest and detailed critique, then you should really get involved with a critique group. Let's put in this way: I assume that when you get an agent you want one who is focusing her time and attention on getting you a publishing deal, reviewing the contract and making sure it's the best contract she can negotiate on your behalf, reading and critiquing your work, helping to facilitate communication between you and your publisher, guiding you with publicity and marketing, etc. I assume you don't want an agent who is spending her time sending out detailed responses to authors who may or may not ever be published and neglecting your work because she doesn't have time.
The only people who deserve my time and attention are those listed on my client list. The best I can do to help other writers is to attend conferences, provide information on this blog, and donate the occasional critique for charity. There are times when I am able to give a detailed critique, when I truly believe an author is close and will succeed with a little push. At other times my rejection could mean almost anything—I'm not the right agent, your work didn't resonate with me, the writing was horrible, the book was clichéd, I have something similar on my list, I don’t like mice. . . .
I would like to see all authors succeed and learn as they go. Unfortunately, I can't make it my job to see that happen.