The result? The most powerful spells in that adventure were not the fireballs and lightning bolts and insect plague spells. In fact, Tomb of Horrors virtually gave away the trick by having very few monster encounters in the entire adventure. No, the "trick" to both of these adventures (well, Necropolis was actually a collection of about eight adventures) was the use of utility spells. And creatively thinking outside the box. And connecting patterns together. Oh, and - if possible - interrogating prisoners (or even interrogating the dead with spells like Speak With Dead).
Things like Divination spells were the chief weapons in a caster's arsenal. The Disintegrate spell, when I ran the adventure, was not used against enemy forces, but was rather used to bypass a really deadly-looking trap. That's right - Disintegrate CAN be used to obliterate stone, wood, and steel too - not just enemies. Or did you forget?