Oh I'm much too much of a teacher's pet not to finish a book, even when I hate it.

I agree that it would be more sensible to quit if the book is no good. The only book I can think of that I've started and not finished is a biography of Yeats. It's a scholarly work, but I was so down at the time I was reading it (not because of the book) that I had to switch to something light and funny. I still have it so I guess that I'll finish it someday. But just as I'm writing this I'm thinking I've probably developed a mental block about it.
The way I protect myself from having to finish books I don't like is that I screen everything before I start reading. I read reviews, the covers, the authors bio, some bits from the beginning, the middle, etc. How I love private bookstores - they're so patient with people like me who have to read half the book in the shop before they buy it!