I find it curious that the quotes posted here have higher votes on average than the usual discussion comments...Why do people value them, I wonder?

I do not conclude that they value them. I think people vote for top-level posts and other stand-alone situations, such as quotations, based on whether they like them, while they vote for comments in on-going discussions based on trying to push them to a particular score, which is usually positive but low. I'm not entirely sure what puts people in different voting modes. Alicorn's comment is surely true, but I'm not sure whether it's an independent effect or a cause of the different voting mode.