Should I backup my Library folder?

I've just got a new HD and I'm not sure If I should backup my

/Library
/System/Library
/user/Library

People tell me If I should,

thanks.

My method is to do a Superduper bootable back up to an Ext HD which i keep off site {covered for Fire/Theft}. When my MBP failed used wifes iMac and booted off Ext HD and continued as normal until MBP came back a week later, then just copied changes to MBP off of Ext HD.