Their not must have Spells, rather their spells that will prove very useful and if used correctly can allow the characters to avoid a tough fight, here's how in those examples
In Sethotep theirs several groups of weak ghouls and even though they can see through the Darkness created by Darkness 15 ft radius the things have almost no intelligence and could use the Darkness the spell creates as a decoy
In Shedu's Hall the foe is A 11th level Magic User whose Hp is well below Average and Dispel Magic can dispel several of his Spells, 5 Spells he could use that The Pcs should try to dispel are Charm Person, Invisibility 15 ft radius, Fly, Polymorph Other and Animate Dead
In The Levels 7 to 9 version of The Mound in The Ring once you've killed at least 6 of The 15 Worgs you could try to use Fear to Scare them away
In The levels 9 to 13 versions of The Marsh Idol after you kill the 4 Fog Giants you could use Animate Dead on 1 or more of the dead Fog Giants
The reason why, because you've still got to face The Villains ally Mikfir whose A Liche that has the abilities of A 14th level Int 20 Magic User, A 9th level Wis 19/Int 15 Cleric and A 6th level Wis 17/Int 13 Druid, A Flesh Golem and The Main Villain Hanyka, whose A Ancient Marsh Dragon, the rules for this creature were in the article as the scenario, that knows 3 1st level, 3 2nd level, 2 3rd level, 2 4th level and 1 5th level Spells