Is it time to start calling out bad cyclists?

Have you followed people on the road that never ever look back? I do all the time, it is worrying when they just stick an arm out and without checking behind swerve across lanes of traffic to make a turn (assuming they even bother to signal).

When learning to drive my instructor suggested that in normal traffic a mirror check every 10 seconds was a good rule of thumb, that's on top of the ones you'd make before a manoeuvre, approaching lights, spotting a potential hazard, etc. Do you advise something similar to your trainees? General observation and awareness of what is around them is woeful amongst cyclists.