Roles Really Aren’t That Important in 3.5
To begin, spells are the most powerful class feature in the game. Dungeons & Dragons 3.5 doesn’t really care much about roles: you will be more powerful the more magic you have. You will never be more powerful going for a non-magic class, even if the rest of your group is already magical. So I call this party a good idea on that account. Unfortunately, you do have some pretty severe power disparity here...
Wizard and Druid are two of the most powerful classes in the game. All else equal, they can seriously handle a lot more than a group of, say, Monk, Paladin, Fighter, Rogue, and Healer could, at all but the lowest levels of optimization.
Duskblade and Dread Necromancer are good classes, certainly capable of holding their own. Both are not-awful without spells (Duskblade being nearly as good as a Fighter would have been, and Dread Necromancer having an army of minions), too.
Spellthief is a pretty weak class, but it’s pretty cool. There’s a Trickster variant from Dragon vol. 353 that’s pretty good (lose all but the first 1d6 of Sneak Attack, a fair few skills, and Trapfinding for Bard-like spell progression and adding the Bard spells to your list), though it may make the character less “theif-y” than the player wants.
If possible, you should make Godsblood Spelltheft available to the Spellthief – select Deities or Domains that are appropriate for your setting. This feat gives the Spellthief some nice reliability: if the spell he steals isn’t so great, he can always use one of his Domain spells. It helps quite a bit.
The Spellthief is also the only character with Use Magic Device on his class skill list, and only the Dread Necromancer also uses Charisma. This makes him pretty ideal for the party’s wand-wielder. In particular, your party lacks access to Cleric spells, which are quite good. Wands require a static DC 20 UMD check, which is possible to hit pretty consistently by mid-low levels. Not all spells are good choices for wands, but judicious use of them can give the Spellthief a lot of options.
You actually do have the four roles...
Despite the fact that you don’t really need them, you do in fact have the four traditional roles. The Duskblade is a warrior-type, the Wizard is your arcanist, Druids can heal quite well with lesser vigor (I recommend he gets a wand of it for keeping everyone topped up between fights), and the Spellthief has Trapfinding and the like. Dread Necromancers provide a pretty good “fifth class” here – minions can be whatever you need them to be, and he’s got decent curses and battlefield control.
Of course... the Druid or the Wizard could very easily handle front-lining (Wild Shape, polymorph, etc.), or trap-killing (summons are great for that), or minions (Animal Companion, Handle Animal, summons), or... just about anything else...
Since this got brought up in a comment, some deals with anti-magic field.
It’s a high-level spell.
It’s always centered on the caster.
It’s got a really small radius.
A caster casts anti-magic field on himself, and he’s done.1 Even if a warrior gets an Anti-magic Torc, activating it kills all his magic items, and now he’s in a lot of trouble too. And in both cases, they’ve got to get within range of a spellcaster, activate the AMF (or somehow get in range without magic, but a spellcaster who lets that happen doesn’t deserve the name), and then do something before the spellcaster simply leaves. That radius is way too small to easily pin someone in it, and actions are working against you here.
Casters should be aware of AMF, and build for it. Conjuration (Creation) spells that have SR: No fly right through an AMF – the Wizard’s got a lot of those in Spell Compendium in the form of the orb of spells. The Druid’s Animal Companion, and all of the Dread Necromancer’s minions, can operate more-or-less unaffected by an AMF. The Duskblade does have full BAB and good HP and armor, which means he’s about as good as any other warrior in an AMF. The Spellthief is really the only one who will be largely sidelined by an AMF.
Dead Magic Zone
This is like an AMF, without any of the problems of an AMF. Worse, it’s not even associated with an enemy – it just is. Just, don’t use these. They’re pretty bad for a normal game (arbitrary and asymmetric crippling of characters, with absolutely no recourse), and for this party it would just be a jerk thing to do. At that point you might as well just “rocks fall, everyone dies.”
1 For the purposes of this discussion, I am ignoring the Cheater of Mystra. If you’re seriously playing at that level of optimization, you’re already well out of my league and I cannot help you.