It would be a PITA having constant intercom hiss in your headphones! Sometimes when the intercom stays switched on, you have to ask the other person to turn it off, it is annoying. I do not like going 'on ears' intercom. It may be quieter, but it separates you from the aeroplane. Sometimes the cabin crew would be standing there and you would just not hear them. A tap on the shoulder and you jump out of your skin!