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This page was missing for a long time, and while the correct part is easily said (the first sentence only), it is not so easy to really explain what a secondary ko threat is, imho. For: when a ko is played with both players expecting it, when it is suitable according to global temperature, there probably is no such thing as a secondary threat, as compensation by tenuki would be bigger than the compensation by a threat not large enough to be answered. Secondary ko threats make sense only in "hot" kos, where moves in the ko are much larger than those elsewhere. Is this about right?
Bill: Ko threats make sense only when the ko is worth fighting. Global temperature is an informal concept. If you make it precise, then when the miai value of the ko is the same as the temperature, the players are indifferent about playing the ko, and fighting it is senseless.
I believe the distinction between kos close to ambient temperature (your typical endgame kos) and kos much hotter than that is worth making, as often secondary threats might not be the best method to get compensation when you intend to lose the first type of ko, but playing tenuki is.
The first type are those kos which are too small to fight when they arise, so you wait until they are worth playing, which is when the moves in the ko are about or slightly larger than the rest of the board, no? Simply because the difference between tenuki and a move in the ko is small there is little space for secondary ko threats. (If there is no difference at all, then playing in the ko might be indeed pointless.) And many players do lose point when losing a ko because they limit their vision to large follow up moves which are together with the follow up still smaller than two moves elsewhere.