[SixOfSpades]

Fighter/Illusionists clearly take the prize in SoA, and even once you hit ToB levels and can finally cast high-level spells, the only Necromancy spell you're going to miss is ADHW. So, as long as you've got at least 1 Primary Wizard who can cast ADHW for you, the only drawback for the Fighter/Illusionist is the lack of Skeleton Warriors--and should your Gnome want a decent Level 5 Summon, there's always Lesser Fire Elementals.
(And it's not like a Tank is going to be casting offensive spells in the middle of a battle anyway; they usually have better things to do.)

Spells that Illusionists cannot use (Good spells in white, decent spells in gray):
Level 1:
Chill Touch
Larloch's Minor Drain
Level 2:
Ghoul Touch
Horror
Level 3:
Hold Undead
Skull Trap
Vampiric Touch
Level 4:
Contagion
Spirit Armor
Level 5:
Animate Dead
Level 6:
Death Spell
Level 7:
Control Undead
Finger of Death
Level 8:
Abi-Dalzim's Horrid Wilting
Level 9:
Energy Drain
Wail of the Banshee