Is it worth having a showreel made from scratch if you haven't done screen work yet?

Hello All :) I'm interested to hear from casting directors how important it is to have a showreel. I've been told that without a showreel a casting director won't even consider looking at an actor's resume. So, if an actor hasn't done much (or even any) screen work that he can make into a showreel, is it worth having a showreel shot from scratch (by professionals), or could that do more harm than good?