I keep my entire ebook library with me on my device - though I must say that it's significantly smaller than the libraries that a lot of you have in your possession! My only real motivations for this are laziness and lack of foreward-thinking: I don't choose what books I'd like to read after I finish my current one, because I'm indecisive by nature and can spend hours deliberating between two or three books, anyway. An extension of choice doesn't spoil it; the choice is hard enough to begin with! Furthermore, I find it much speedier and more satisfying to press ctrl + a and then 'Send to Device'. It saves having to scroll through my separate Calibre libraries, spend hours considering what I'll want to read, and then shift + select numerous titles to send.