I get really annoyed if even my books that were lent out to friends were returned in a crumpled condition. It happened exactly once over a decade ago, and since then our friendship has fallen on the wayside. How a person treats belongings that aren't his/her own says a lot about him/her.
And since my bags are thousands of times more expensive than books (and more rare/difficult to replace), I have no problem saying no. If I'm being kind, I may offer a reason, but you certainly aren't obligated to do so! Lending someone something is doing them a favor, not an obligation!