All things being equal (money, space, etc), would you rather own copies of the books you read? Or borrow them?
I’m assuming I have a TARDIS-like home here where I can have infinite rooms and corridors full of books…LOL.
That said, I still prefer to borrow for the most part. A lot of times, I’ll see a review or hear about a book and think, “That sounds interesting, I’d like to read that” and then I get it home and find it’s not quite my cup of tea. In that case, I’m not as disappointed if I didn’t buy it but only lost the time I invested in reading it. And I can then put the book back into circulation for someone else to read and potentially enjoy.
I do have some authors and series that I want to have on my shelf. Some favorites I enjoy owning and displaying after I’m done reading them.
Nashville Book Worm 
Good answer
http://tributebooksmama.blogspot.com/2011/06/booking-through-thursday_09.html
I am all for selective hoarding. I already own more than 2500 books!
Here is my BTT: Own or Borrow post!
I’m a big re-reader so I like to own my books.
Sally.
http://theelifylop.blogspot.com/2011/06/booking-through-thursday-21.html
I’m much the same as you, there are some books I have to have and others that I’m fine just borrowing. As you say, not every book is worth investing more than a little time in.
A Tardis-like home sounds just great, or maybe a home like the tents in the Harry Potter series. I’d think I’d like the Tardis more, time- and space travel sounds just great.
I am a keeper until they get out of control… then I give some away but I don’t like to borrow books at all.