Insurance is ONLY correct when the probability of a dealer 10,J,Q,K is >= 1/3.

Consider a single deck card.

Without any additional circumstances that probability is 16/51 - which is too low.

But if the player had 2 Aces - then split to receive a 9 apiece:

The probability of a dealer 10,J,Q,K = 16/47 - higher than 1/3.

And the count is -3.

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I haven't played this millenium so my memory of the rules and strategy is rusty.

But assuming the player can insure at any time while completing his hand there would be a number of similar situations MANDATING insurance.