Thanks for the replys thus far, I hope a few more people reply to give a more general idea of the problem (if there is one) and other peoples experiences.
For the record my AME thinks airlines are boardering on being negligent with regards to health and safety relating to this apparently common practice.
As an experiment I suggested to a captain I flew with a few days ago that we "try" the hot mic, he was keen and we had a very pleasant flight with a reduction in volumes of approx 20% with no missed or mis-heard calls.
Can somebody offer any sensible reasons FOR using the one ear off method ???