Arcane Warriors can use their sustained abilities to get very high levels of defense/armor/spell resistance (at the cost of extremely heavy fatigue). This can be powerful, but limits what the Arcane Warrior can do to basically being an auto-attacking machine (decent damage, great defense).
Some people prefer more of a 'hybrid' build. Meaning to engage in melee while throwing out cones of colds and the like. However, many spells cannot be cast with a sword in your hand. If you try to do so, your character will sheathe the sword, then cast. This delays the casting and can cause a lot of trouble.
This behavior is controlled by a flag called autodraw in the ability table abi_.gda. A flag value of 1 means the character has to unsheathe their weapon and assume combat stance before the ability can be cast (for example: Berserk, War Cry). A flag value of 2 (used only by spells) means the opposite - the character has to sheathe their weapon first, unless the weapon is a staff.
Aside from directly modifying the abi_.gda file, there is a work around for this behavior: switch weapon set right before casting a spell that requires sheathing weapon. When the character switches to their alternate weapon set, the weapon will always be in sheathed state, allowing the character to immediately cast the aforementioned spell. Pausing the game is recommended for this maneuver, since switching weapon set can be done when the game is paused, and this makes sure that you can issue the cast command before the character unsheathes the alternate weapon.
For those who don't want to use this mechanic, you can build your Arcane Warrior around those spells that can be cast with a sword in hand. A potentially viable alternative is to simply punch targets, keeping a shield equipped for the defense bonus. The shield will remain on your back, leaving you free to attack and cast spells as you wish.