One of the more common reasons I've heard for downloading a copy is the "just looking" one. On the one hand, I get this. One of the things you can do in a brick and mortar store is wander over to a chair, sit down, open the book, and start reading. Read the blurb, the critical praise, find out about the author and take a gander through a chapter or two - and then make up your mind. I see wanting a parallel here.
But, here's the thing: I'd say most of the time, there already is one.
Let me try to do this with a few books in which I have a story.
So. Finding those took me all of a few seconds each - that wasn't work. Granted, I knew what title I was looking for. It's not quite the same as a general browse, but neither would the file-sharing sites be (though most of the websites listed there have the "people who liked this/browsed this/bought this also liked/browsed/bought..." links on the page, too). Big publishers make it even easier (take a peek at Random House, or Tor to see two great examples of easy-to-browse peeks inside books).
I'm not sure I've ever needed more than the first dozen to twenty pages or so of a book to figure out if I want to read it - maybe that makes me odd. But I will say that when a free glimpse is available, I really don't see the validity in "I just want to look at it" as an excuse to copy and download the whole book.