I have only seen a run and break performed once. That was at Bournemouth, which is in controlled airspace, and the manuover was flown by a Vampire and no other traffic was anywhere near the curcuit. Seemed a very sensible way to get the speed off and positioned correctly on downwind considering the way the aircraft approached the field. However, I do not see why this manuover is needed apart from the enjoyment factor. A bit of forward planning would remove the need.
So here we have one group saying it's a good practice and the other group saying it's dangerous. I know that I would feel very uncomfortable being in the circuit if somebody called their intention to do a run and break so I must fall into the latter group. Is airmanship something that only low houred pilots need in order to avoid more experienced pilots that insist on doing these manuovers? I consider my safety to be more important than your fun.
D