On any list of writer's complaints you'll find "no feedback on rejections" near the top of the list.
Most writers understand why agents don't offer feedback, or anything more than a form letter on queries. They understand the time debit of personal responses.
Start talking about partials and fulls though, the attitude changes. I understand that. I do.
But, here's what I do instead: I read more fulls.
Writing personal responses, analyzing, taking notes, writing an email; all those things take time. I elect to invest that time in another way: reading a manuscript.
I understand a form rejection of a full is galling, but think of it this way: I asked for and read your manuscript because I didn't spend time critiquing the other guy's novel.
The only place I critique work is the Query Shark blog.