I've come across this using "old-work" boxes, but all plastic boxes for switches and outlets are basically the same:
These boxes have 4 knock outs per gang and they open at an angle. They seem to be made to allow the wire to enter one way and prevent it from slipping out like a basic clamp.
From time to time, one of the plastic tabs will break off leaving an open hole. I've also found installed boxes that have the tab missing.
So, is it OK to use the box with a broken tab? If not, to what level is it "not ok"? Is it a best practice, or is it to the level that an inspector would make you replace it?