It's a chicken and the egg scenario at best. Obviously, there isn't one right answer. I find that it depends on the type of book. I buy textbooks because I have a slew of kids and odds are someone else will use it. If not, I can re-sell it and recoup the money quickly. I buy books that we love even if I was gifted an ARC. It's my way of giving back.
I don't buy books we "like" as in, "That was good, or ok" Those books can sit at the library and wait for us. I don't buy new release big name books anymore either. I'll be able to pick up the latest Patterson or Grisham at the library sale in hardcover next Summer for a quarter. I can wait- it's probably going to be a movie anyway- right?
You own it. It's always there. Finding it on your shelves may not be easy- but it is there somewhere.
Immediate Gratification is no small thing when it comes to new books. You can be reading that new release book the day of publication especially if you've been waiting for a sequel.
The Re-Read. Each Autumn I re-read all or part of the Harry Potter books. It's a tradition.
Sometimes buying is the best financial choice. Have you ever checked the same book out of the library two thousand, million, times and then either spilled on it or lost it? I have, and then I ended up buying it anyway. If you or your kids love a book- buy it. Library fees add up, and Library versions are more expensive than regular hardcovers.
Write a dedication or memory in the front of books that you love. I found some books that my great grandmother wrote in and they are the only things I have from her. Do that for your kids and grandkids.
Buy classics and favorites in hardcover. If you have multiple kids who love that book, you may want to consider multiple copies so that they each have one as adults.
Library Trips. A weekly trip to the library is a fun and useful tradition to start with your kids. I use this list from Susan Wise Bauer and have each kid pick up a book in each category:
History, Science, Fiction, Poetry, Art, Craft, Biography.
Deadlines. Yes, books can get renewed. But, there is something to be said for a deadline. I know if its a library book I have to read it while I have it rather than letting it sit on my TBR shelf for months.
No Buyer's Remorse. You can ditch that book at 3% and not look back. Yes, you can borrow E-books from most libraries which make it even easier to decide that you aren't finishing it. Just hit delete and move on.
You won't break the bank. Even if I were Bill Gates I wouldn't be able to buy every book that interested me. Unless I had that library from Beauty and the Beast- Hmmm.
Librarians are awesome. I was even more convinced of this after attending ALA. Librarians know books, they know old books and new books and can lead to books that you'd never find on your own.
Not sure if you'll like that book? Borrow it and find out. Thinking about buying a boxed set as a holiday gift? Borrow the first book and see if your kids love it.