So how can I possibly say that rejection is ever a good thing?
You could somehow have found yourself in a situation where you're querying a manuscript that's not in the genre you want to debut in (I have done this before--in that I realized I wanted to debut in a certain genre and what I was querying at the time wasn't that genre. So I stopped querying.) You also could find that while you're waiting the long months for agents to get back to you on a full (or to hear from a publisher at all) you think of a really brilliant but seriously hefty way to revise the novel (that might change its genre or age group), but it'll take some time or you're in the middle of something else.
Rejection still hurts, no matter what. But it's GOOD if a book that isn't where you want it to be gets rejected. If you're like me, you don't want to start off your career with something that isn't for the age group or genre you plan to be writing in for the majority of your life. And you don't want an agent or a publisher to want to sign you, but when you tell them you'd been thinking of certain revisions already, they say they're not into that idea, or they only want minor changes. Can you say no to a deal you've dreamed of because your novel isn't right by YOUR standards? Would you even think of saying no?
I can think of two more examples: when a couple months after an agent rejects you, he or she leaves the business. And when you realized based on what an agent said when she rejected you (if it's not a form letter) that she would be wrong for you anyway (she doesn't like main characters with an interest in unicorn breeding and you can't imagine not writing about characters who breed unicorns.)
So what about you? Have you ever realized receiving a certain rejection was a good thing? Can you think of any other situations in which getting a rejection would be the best possible outcome?