Cover First of Equal Honors?

A Q 10 3
A Q 8
J 10 6 4 2
A
8 6 4 7 5
J 10 9 5 3 7 6
5 K Q 9
K 10 5 2 9 8 7 6 4 3
K J 9 2
K 4 2
A 8 7 3
Q J Contract: 6 spades

Just a day or two ago, I discussed the situation where a defender, holding K Q 4 in trump, split his honors, against an apparent 9-card suit, on a lead toward the ace, which I termed ill-advised, given that you can't be skunked out of one winner in trump (as you might in a side-suit) and given that declarer with 9 trump might well bang down his ace and continue the suit, giving you two winners. At the time, I wouldn't have bet even money that I would find a K Q 9 sitting over a J 10 x x x in a year of looking. And there it was, a day later, or at most two days.
When do you cover the first of equal honors? Well, one reason for doing so is that you have as many as dummy holds and there's a reasonable chance your side has the card underneath that might be promoted. Here, though it's not the trump suit at issue, that "reasonable chance" is a certainty when the key cards all lie with East. On other occasions, when you don't see the 9 and there's a reasonable chance your partner has 3 or four cards in the suit, then also, I would cover the J and 10 with the K and Q.
Here, of course, failing to cover the J 10 with the K and Q can only be attributed to inattentiveness. There is absolutely no reason not to, and the slam contract would thereby be defeated. Twelve people made six spades here and one made 6 no. That seemed like an extraordinarily high number. But on a second look, I saw that ten of those thirteen declarers were playing from the North hand and got the king of diamonds opening lead, and I'm not here to second guess an opening lead, often called the most difficult decision in bridge.
But that still leaves three cases where a defender, looking at K Q 9 sitting over a J 10 6 declined to make sure he covered each of those two honors. And the remarkable thing is, they found two different ways to throw away that trick. Two defenders went up on the lead of the deuce! They didn't see that the 9 would draw the ace (or win)? There's no other card that can beat the nine. And the third defender played the 9 under the 10.
I dunno. Yes, you cover the first of equal honors when you have as many yourself, and indeed, this is true for a holding of A Q x over J 10 x also, though I won't go through all the permutations of how that too can promote a nine.