I am torn. Young adult literature mostly sucks and is a waste of time (not all!). However, the adult fiction people tend to replace it with also sucks.
So it's an operational definition? A book is a young adult book if it goes in the young adult section of the public library? Someone needs to do an edition of Justine with adorable illustrations under his full name rather than his title. See if Young Adult de Sade happens.
Yes, and it's one that shapes how children read and how various education programs are structured. (Common middle school assignment: "pick one book in the young adult section and tell us what you think about xyz...") Therefore it's worth examining as a category. You can say it's meaningless all you want but it has a negative impact on what kids read. I worked in a library for several years and repeatedly saw kids anywhere from 10 to 17 work their way through the young adult section without it ever occurring to them that they could ever read from the adult fiction section. (And as you say the distinction was ridiculous, and they certainly could have: the first shelf of the first section of the adult fiction section contained Watership Down and Mostly Harmless.) EDIT -- I should add that I genuinely appreciated your smirking reference to the Marquis, though.So it's an operational definition?