In "Mixed Reviews," I ranted how it's mind blowing agents can decide on whether to see your manuscript based on a query (and possibly sample pages). Those rejections I can handle without seeing it as personal (I received five in the past two days and I'm still standing-unfazed). Agents (and gatekeepers) are bombarded with writer hopefuls, praying that someone will find interest with their work. But when a agent finds interest, requests more and than rejects you, how can you not take it personal?
Take this analogy if you will. To me, the query letter is like an annoying door-to-door salesperson ringing (and/or knocking) on your door. A request for a partial/full manuscript is the same as offering that person into your home. Do you know how rare access is? So once you get inside it's up to the salesperson to convince the homeowner to purchase what he/she is selling. In the world of publishing, once a substantial sample of your work is in an agent's hands, it's your writing that will have that impact necessary to obtain representation.
There are many reasons why an agent won't offer representation. Either he/she didn't connect to the material. The writing sucked. The story sucked. Too much to revise. Not easily marketable. Unfortunately, the agent most of the time gives a polite form rejection letter that usually implies that the manuscript wasn't the problem, but the agent's enthusiasm towards it was. It's (again) polite and encouraging, but still gives the author hope that one agent will have that fervor the others lacked. But, if the writing and/or story sucked, an agent's acceptance will never come. For some that might be okay, but for me I would like to know.
If a writer's growth is based on how he/she progresses through revisions and further projects, then it would be helpful to know an agent's honest opinion. Especially, if she/he thought it was worth reading. I'm not asking for line to line analysis, just one's overall perception. It still can be polite, just straight to the point. Why should I waste another agent's time if the manuscript is completely doomed? Besides, I'm still going to possess the same thoughts with a formed rejection then I would without being told the truth.