If you don't want to guard a door, don't put yourself in a position to guard the door. If I'm guarding a node as a healer and we need heals elsewhere I just say something in ops chat and someone relieves me. Your job as a "guarder" isn't to solo kill everyone who approaches you. Your job is to keep them off of the objective until help arrives. If you can't do that you shouldn't be on guard duty.
This. It's about interrupting an 8 second channel until help gets there, (unless they're pew pewing and can't be bothered). I'm a fan of "2 dedicated node guarders per node with 4 floaters" idea, or 2/6 in the case of VS, because it's not uncommon that a team will over-rotate and lose the abandoned node.