A report highlighted by Times Ideas suggests that highlighting information as a form of learning has little real impact over simply reading the material. Yet, the evidence (and my own experience) is that this a very popular way to aid 'learning'.
The truth is that we do many things because we believe them to be effective, rather than because there is evidence that they actually are effective. I wonder what else we do that provides us with a false sense of security … oh, yes, handing out copies of slides. That too doesn't really help.