I think that is naive. You'd want to know everything you can about him - height, build, clothing, colour, so that you can be sure you have the right person when the confrontation arrives. If he's disposed of, or hidden the knife within his clothing for example, then all those descriptors are vital. You can't rely on him swinging the blade around when you arrive for identification.
Now I think about it, that is correct, every time we have a robbery we always rely on immediate identifiers, bit thick of me, I blame the vodka. I was just concentrating on the negative connotations