- The book will not be available in the future (out of print, etc.). Fair point, but uncommon.
- You think acquiring and storing unread books during your working life is more convenient than acquiring books on an as-needed basis in retirement. This is unlikely, since (1) you will be home all day for the UPS guy then and have unlimited time to potter in book shops; (2) you may also be paying higher rents now than you will then, if you currently live in someplace that's pricey due to good schools/proximity to the workplace. Storing stuff has costs.
- You think the future value of the money you spend on books today is less than the cost of those books in the future. I don't anticipate book price increases beating market returns.
- You like having lots of stuff.
To disabuse some mistaken visitors: I don't think that unread or bad books cannot touch a shelf and must be hidden in boxes under the bed. I just don't find it desirable to display them on more prominent shelves. I also don't have a problem with having a few unread books in the queue; I do think it is a waste of space and money to buy so many books that you will definitely not read them all. My scorn and distaste really are only aimed at people who buy books because they like to be perceived as the sort of person who has a lot of books.
This reminds me: I need to put up or shut up re: that D.H. Lawrence and the biography of Learned Hand.