No, I think people being stupid teaches people bad habits regardless if is a plastic gun or not.
Can you please explain to me why an instructor would need to walk in front of the firing line wile people are shooting other than the fact that he is an idiot who will probably end up doing it one day during live fire.
To answer your question I think they are training aid good for teaching people who have never handled a firearm before, I think they're a good tool for teaching CQB and how to deal with armed opponents. I do however feel that if you cannot obey the safety rules with a plastic firearm you have no business using a real one.
Do you think it's ok to muzzle your partner wile clearing ? What about muzzling yourself wile exiting your car ? Is that ok because you know it's only a toy gun.