I find it completely optional. I run premade adventures ( no time to craft my own). It would be very useful if I wanted to create my own world. Also, my gaming group is trying to adjust from 4e, and they all said that they don't want to try out any of the optional rules until they get the hang of the PHB rules. I do intend to write out combat action cards, like disarm or mark, and make them available to the group.