I just had to post one more reason why I own books: Because I abuse them.
I read in the bathtub, while cooking, while doing dishes, while gardening, etc. I can't treat library books like this. So I tend to buy a lot of used books, keep them for a couple of months while I read and abuse the poor things, and then sell them back to the bookstore. Essentially, I treat the used bookstore like a library.
But, again, I see no fundamental problem with owning very few books and getting most of one's books at the library. I don't see how any home could have a large enough variety of books to keep a voracious-reader child satisfied.
Miriam, if you want to get rid of the books, GO FOR IT! I've only got about 70-80 books, but aside from reference/history/classic fiction, the majority are obscure, impossible to find in the library. I'm moving soon and donated a ton to my church's library. The suburb I'm moving to has an EXCELLENT library!: I want to read more history and libraries are great for that sort of thing. I've decluttered TONS of books over the years.
ETA: Have you read A Tree Grows in Brooklyn? The heroine was in a very poor family growing up, but she always got tons of books from the library.