Dennis, You are correct, having a sentinel is no substitute for more scope or more chain. I never said it was. But there are times when scope is limited due to lots of factors (re: Anchored in a Harbor with many boats to watch fireworks, etc). I guess I could do as you say and just move on and leave, but if you wanted to stay any added weight can benefit. I am not saying I would add more and more sentinals. If you do that, then you might as well just go to all chain. So I am agreeing with you especially in gale and storm conditions. But not every night is like that. And yes putting two anchors in tandem is probably the ultimate from what I have read in cruising guides, but I have also heard that they are a bit of a pain to deploy, set and retreive. (Though if there was a huricane coming I would definitely consider doing it).
So Dennis I agree with you and I said so in the first sentence
"... nothing can fully replace adding more scope or having an all-chain rode."
But deploying a sentinel can only help, it can''t hurt.