Its pretty hard to nick something in full view of everyone in the library you know. Phones and wallets I understand, and I take mine with me. But my macbook, not really. Decency wouldn't allow the people around to let a thief get away with taking it. I don't ask people to look after it either. Worst case scenario we have to check cctv but the way things are electronically monitored these days I'd find my macbook in a few seconds. You'd be a bit of a wally to steal a laptop from a full library.
If its quite empty however I will take all my stuff.
To be honest, if it bothers you so much, why don't you steal it? Don't want to? Mind your own business :s