I got told my by my midwife that you should carry them regardless but more so if you work far from your home or have a long commute incase you are involved in an accident and are unconscious and unable to respond to simple questions on the state of your pregnancy.
Since then I've carried thema round relgiously for both pregnancies as I have horrible train and tube journeys every day. Consequently they are very grubby, dogeared, and look as old as me - but am too nervous not to carry them around incase I'm in a train crash or pass out on the tube!