As a kid, I had both. The set of just quasi-random pieces and a large book with tons of ideas of what you could build with it. I also had some typical spaceship ones.
After I build things from the books, they would get taken apart and I would put the pieces all into the same box and just build whatever from there.
My daughter on the other hand, who has been a crazed Harry Potter fan for a few years, has HUNDREDS of dollars worth of Harry Potter Legos. They are all assembled and being displayed on a shelf in her room. My fear is that if they ever came out with The Hunger Games Lego stuff, she would buy that stuff too.
Maybe the idea of just being creative and building stuff is why so many people I know who play any sort of "computer games" like Minecraft. You build entire worlds out of blocks. Sounded so lame until a buddy bought me a copy. Now I look forward to the time that I get to go online and dig sand, put it in the furnace to make glass blocks, turn glass blocks into panes of glass, and then continue constructing my massive glass tree-castle... But I digress...