If the slides do not convey the material that is being taught, then the slides are useless. Students generally do refer to slides after lectures, and usually end up staring at a load of incomprehensible nonsense.
The instructor-expert can add flavour and understanding to the slides, and expound upon the points being made, but the slides should also stand alone as a resource.
A lecturer should appreciate that it's impossible to absorb a complex topic during a lecture, and provide resources that later complement what they are saying.