Hi Jon.
those cell protectors Thornes sell are meant for when a bad queen is removed and a ripe Queen Cell is to be put in soon after so it hatches before any the bad bees cells they start are ready
I havent ever used one but thats what they were originally for.
I think the foil is just a modern substitute.
Snelgroves Introduction of Queen Bees has the full lowdown on when they make sense
Possibly folk take an unnecessary belt and braces approach