I shop online a lot and I use some of those sites all the time to compare prices and gauge the respectability of sellers. They've helped me avoid the "bait & switch" type vendor time and again.
I agree that they shouldn't necessarily rank high every time for a product search though.
Back to the old commercial/non-commercial serps argument. I wouldn't expect Google to do it, but that givemebackmy... site could add buttons to give users the choice.