At the conference last weekend, an attendee raised a complaint to agents and editors when we were all sitting together on a panel. Rejections letters he received over time were usually impersonal - without any comments as to why the story was rejected; sometimes the letters were often xeroxed to the point that the text was barely legible, if at all. He felt insulted by these letters.
Various agents on the panel took turns addressing his concerns. The consensus was that agents do not have time to personalize rejections in almost any way. Occasionally, agents will have "checklists," where they will check a box saying why they rejected something (e.g. "POV confusion," "I don't handle this category of fiction," etc.).
Miriam Hees, publisher of Blooming Tree Press, said it like this: She was spending so much time trying to personalize rejections that her slush pile contained material more than a year old. She changed her policies, figuring that a cold, impersonal rejection was more compassionate to a writer than keeping their manuscript for a crazy amount of time. Makes sense to me.
That said, everyone agreed that rejections with the wrong book title pasted in, or rejections that were not readable, were not fair to writers. (To avoid the latter problem, use e-queries where possible!)
While we're talking about rejections, it's important to mention that more and more agents have adopted the policy of only responding to queries if interested. Usually, they are upfront about this policy and their reply timeframe. If they say "We will reply within 6 weeks, only if interested," you should probably wait eight to be certain. After that, consider it a "no" and move on.