Why put both yourself and the cars bearing down on you from behind at risk?
As I said, if there is room and space, I will do my best to move over and make things easier. But this is very often impossible when the road is busy with both/all lanes full of traffic, as the OP said. In that instance, switching lanes can be the more dangerous manoeuvre - you should at this point be checking over your right shoulder, when really it would be better to concentrate on the road ahead where someone is trying to join...
Better to maintain you speed, make sure you are leaving enough distance between yourself and the car in front, and leave it to those joining the motorway to slot into the traffic flow.
Yes it's a courtesy to move over to allow people joining a slip road more room, and it's one I try to extend to others at every available opportunity, and one I appreciate from others - but it's just that; a courtesy, and nothing more. At the end of the day, it is down to the person joining to give way to traffic already on the highway, and that traffic cant be expected to start changing lanes for you.