Most of the time but depends on the airspace and operation. Not always necessary but particularly around some of our Southern California airspace it's nice to be in the know.
I always have in the case with my gear even away from airspace in the middle of nowhere I've had crop dusters come in fast & low, so I have it just in case I need to let them know I'm there...
Yes but most people (in the UK) will be unable to transmit and only listen unless they hold an amateur radio license.
Still a good piece of kit to have in the toolbox