Ifixit,
actually i've seen many #1 Rec's and 2 and 5's as well test good with a meter, or even a handyman tester in normal mode and still be faulty. Truck would run but still fault out intermittently because of an internal fault inside the Rec. The handyman tester has a 'leak test' mode where you can check for bleed back through the rec and this usually tells the tale. When you run the test if it turns on the leak light of course replace the rec, even if just blips for a second thats an indication the rec has issues. So the notion that it either works or not 'on or off' is a misnomer (misconception). Just a couple milivolts of feedback can cause one to fault and may not show up under normal testing.
In a perfect world or from a trainer's view though you are correct, most of the time it's either on or off, working or broken ;o).