When sorting through the MANY MANY free promotional books that come up, I definitely judge them on their covers. If someone recommended a book to me, I wouldn't judge it on its cover.
And for all you indie authors out there: I urge you to spend some money for a graphic designer to make a nice cover for you. I guess it's the publisher's responsibility, but if they do a poor job, can you supply your own?
As for my Calibre library, I definitely dislike having no cover and I try to download them from within Calibre. But if Calibre comes up with nothing, or something ugly, it never occurred to me to go out (on Google) and find my own. I don't think I should be spending that much time on it.
And as much as I love Sony's ability to set your own sleep state image, I am a bit jealous of Kobo's ability to display that current cover as the sleep state image.