1. If you get enough coffee table books, you won't actually need a coffee table anymore.
Table by Richard Hutton
2. If you don't like the book, it's easy to give it away, Freecycle it (or just plain recycle it, for those whose veins are icy enough to rip the covers off).
3. Two mass market paperbacks cost less than $20 (see School Library Journal's average book estimates for 2009). And books on the library's book sale cart are only $0.50 or a $1 each.
4. You can take it on a plane. If the book is engrossing, you'll stop worrying about the child next to you spilling his apple juice on you. If it's not, you can leave the book in your seatback pocket.
5. The more "good" books you buy, the more "good" books will be published (and the more the ghost-written photo-filled books, calendars, glittery thingamajigs and other impulse buys in the bookstores will recede.)
Feel free to post more reasons in the comments.