I only let drivers out of side roads if the traffic on the main road is stop-start or slow moving. If the traffic is flowing normally, braking unexpectedly could possibly cause a problem for an unaware driver behind.
My driving instructor warned me that doing this during a test would not impress the examiner, it would result in a failure. Some drivers will wait for 2,3 or even more cars, just causing a queue of traffic behind.
On my usual route home there is a building at a junction where the car park can be entered from a side road and exits on to the main road. Drivers often use this to cut the corner and jump then they try to force their way out into traffic. To me they are simply queue jumpers.