It's the job of the ground operator in almost any RT environment to control the flow of communications, if the rest of us start bypassing them without immediate and pressing reasons (you've spotted something endangering an aircraft, formation calls, message relay), then in my opinion the result will be chaos.
The two (civil) exceptions I can think of are traffic calls and where an aircraft is co-ordinating a SAR operation. In both of those, there is nobody on the ground.
G