If I'm inbound to an airfield and there's someone in the circuit but no-one manning the ground station, I EXPECT the other pilot to pass me any relevant information he might have. To remain mute is just plain BAD airmanship.
On the other hand. If I'm manning the tower and an inbound pilot decides to cut me out of the loop, I'll be just a little miffed.
As for an A/g operator not being able to issue instructions: Sometimes you have to in order to avert an accident. There just isn't time to make a polite suggestion. Stuff the rules.