There are a lot of dumb prestige classes, but I usually can at least imagine some use for them in a campaign world or see why someone would want to play one. And then there is the Rage Mage from the Complete Warrior. Really, what is the use of this and how does it make any sense? Let’s take a look.
The requirements are sort of all over the place; must be able to rage, cast 2nd level arcane spells, have Combat Casting, and a Base Attack Bonus of +4. So a character must be a barbarian and most likely a sorcerer (I am going to rule out a wizard, it just makes no sense to have a wizard that rages, I think they would tear apart their spellbook). Generally a character that rages is up front charging and cleaving, while the spellcaster hangs back. So this is an unnatural marriage to begin with. To get into the class it takes a Sorcerer 4/Barbarian 2 (that seems to be the quickest route) that has used one it’s three feats on Combat Casting. That in itself is a pretty weak class, but it gets worse with the prestige class.
The basics of the class are d8 Hit Die, 2+ skill points, 1 good save, and BAB as a rogue. Overall that is pretty weak, so I expect the class features to make up for it. Not the case. Spells advance every other level (starting at level 2! I assume to prevent people dipping in for 1 level to get the powerful Spell Rage) so it is a weak caster. It’s main ability is the Spell Rage, which allows the character to cast a spell while raging using their full character level (this ability is limited to 5 schools of magic). As they advance they get a small ability to overcome spell failure for using armor (10%, so they can safely wear leather!) and an increased DC of 2 for some spells. The latter is the awesomely named Angry Spell. At 10th level they can cast Tenser’s Transformation as a free action. Remember this is at least a 16th level character. A wizard could cast that at 11th level.
As a DM I like the image of a spell caster rolling with a band of barbarians, killing and looting with reckless abandon. In fact, I have even used this type of NPC. It’s called a Neutral or Chaotic Evil sorcerer. I don’t need a prestige class for that. Instead of a rage he can just get drunk or crazy and not involve a game mechanic at all.