Thanks to all for the answers!
Let me to specify the contest: I'm actually employed as flight instructor in Europe. After discussing similar videos proposed to me by some students, I gave to all of them specific order to avoid adopting such behavior according to Robinson 22 Safety Notice 17 and to EASA provisions (mentioned above by rotarywise) for their future employment in other helicopters. I personally never left the pilot seat while performing solo flights with rotors engaged-engine idle and no one of my flight instructors in the past were favorable about the issue.
So, are you still really sure that a similar action could be considered an acceptable habit?
What would you say to your students about it?