Is it time to start calling out bad cyclists?

My cycling mileage isn't much, but I've shouted to another cyclist in recent times who had decided to go halfway down a large truck which was in the process of turning left. He got up on the kerb and avoided being squashed and was grateful enough to give me a two fingered salute before continuing his journey. I almost never call out to any other cyclist, simply because I feel it makes a damn bit of difference.

I get the feeling that its just frustration. When I'm called out on a stupid mistake, I have to bite my tongue. It doesn't necessarily mean the guy hasn't valued your input, it's just he doesn't know how to vent his anger with his own stupidity.