jenlynb, who knows whereof she speaks, has written an absolutely splendid post about why YA books are awesome, and why having a YA category in the bookstore is useful and worthwhile regardless of what age you are as a reader. I heartily agree.
Though I'd extend that to MG too, of course (says the woman who is currently in the middle of reading Starcross, and still giggling over Philip Reeve's description of the planet "Abnegation, which was woven out of brown string by Presbyterians").