I kept all of them them from the various different degrees i started but didn't finish. I have everything from Aeschylus and Ancient Greek history to books on organic chemistry, quantum mechanics, corporate finance, and optimization modelling.
Plus i have a ton of books on programming and systems administration. I keep all my books but I throw-out everything else. I don't keep all the junk most people accumulate over the years.
Fwiw, they're nice to have around. I'll even go back to the organic chem once in a while, not because i need to, but because i forgot how something works and I want to remember for my own sake. Same thing with my history books. The accounting and finance books can be of practical value to just about anyone imo. There's certainly no harm in hanging on to them.
edit: i'm incinerating the couple of poli sci books i purchased. Total crap.