I believe the most common reasoning behind purchasing a hardback (assuming that the paperback is available and this is an option) is the belief that 'it makes a nice gift'. And indeed it does; its size and weight give it a certain sense of bookish importance. The inside cover page of a hardback also lends itself rather nicely to being a canvas for an author's signature.Why should anyone opt for a hardback book? They cost more than a paperback edition, are heavy, and if they are glue-spined (as most contemporary hardbacks are) they are difficult to open and their pages come loose after you have struggled with the book a couple of times.
However, all this fails to counter the fact that once the hardback has fulfilled its role as a gift, it will then return to being a mere book, and must be read as such. And as an artefact intended for reading it is rather less user-friendly.
The biggest drawback of hardbacks is that they don't commute well. They are too huge and heavy to carry and if you happen to knock a hardback against one of your fellow commuters in a train you would be lucky if you got away with only a dirty look and a few choice words muttered under the breath. A good amount of reading on a commute happens with the book in one hand while the other hand grasps a handle or a strap for support. So unless you have wrists and fingers of steel and well-muscled arms and shoulders their is little hope of reading the hardback. Most, actually nearly all, of my book reading happens in trains on my daily commute to office and back home. I have a hardbound copy of Vikram Chandra's Sacred Games still gathering dust on a bookshelf.
Hardback books are primarily a punishment for wanting to read a book as soon as it comes out. Sometimes, like in the case of Harry Potter, waiting up to a year before the paperback version of a book comes out - is simply too long a wait. Am I glad that (like last time) the seventh Harry Potter book hits bookstores on a Saturday and I can read the hardbound comfortably at home over the weekend.