Don't get me wrong; I don't have a problem telling someone that I thought a book I wasn't able to finish was absolutely horrible (and why) if I'm asked. But I'm personally uncomfortable giving them an official "score." I usually just toss those books onto my "Unceremoniously Dumped" shelf and grab another.
I harbor no ill will towards those who rank a book they didn't complete, I just can't do it myself.
