I was looking to roll a new Arcane Warrior to start an all mage play-through of all three games and I became intrigued with this issue of to dex or not to dex.
I am basing the calculations on a Level 14 Elven Mage with bonuses from Into the Fade. I chose level 14 because this would probably be the earliest I would switch to melee with the acquisition of Shimmering Shield. For the sake of simplicity I am not taking into account bonuses from tomes, equipment, buffs, debuffs, etc.
Also, this is strictly from a melee standpoint and not taking into account the defense benefits of dexterity or spellpowers benefits to casting spells.
Attack = Base (50) + .5 * [(STR-10) + (DEX-10)] + Attack Bonus
Attack Bonus from Combat Magic = 5 + (Spellpower/5)
Scenario One - All points into Magic Attribute
Strength 14 (Starting attribute of 10 + 4 Fade Bonus)
Dexterity 14 (Starting attribute of 10 + 4 Fade Bonus)
Magic 63 (Starting attribute of 17+ 5 points at character creation + 2 Fade Bonus + 39 at 3pts/level)
Spellpower 53 (Magic-10)
Attack = Base (50) + .5 * [(STR-10) + (DEX-10)] + Attack Bonus
Attack Bonus from Combat Magic = 5+(Spellpower/5)
Attack = 50 + 4 + 10.6 = 64.6
Scenario Two - 30 Natural Dexterity + Rest Magic
Strength 14 (Starting attribute of 10 + 4 Fade Bonus)
Dexterity 30 (Starting attribute of 10 + 3 points at character creation + 4 Fade Bonus + 13 at 1pt/level)
Magic 47 (Starting attribute of 17+ 2 points at character creation + 2 Fade Bonus + 26 at 2pts/level)
Spellpower 37 (Magic-10)
Attack = Base (50) + .5 * [(STR-10)+(DEX-10)] + Attack Bonus
Attack Bonus from Combat Magic = 5+(Spellpower/5)
Attack = 50 + 12 + 7.4 = 69.4
Based in the Wiki: When the attacker's Attack value is the same as the defender's Defense value, the hit probability is 54%. Each point of attack increases this by one point
So as you can see from the calculations our level 14 Arcane Warrior with 30 natural DEX will hit an enemy with a Defense Value of 64 roughly 59% of the time with a melee weapon as opposed to 54% for the AW with all points in magic.
This does come at a cost however, per Combat Magic: Spellpower is now used in place of STR for damage.
Looking at the formula for weapon damage Damage = Base weapon damage + Attribute Damage + Character Bonus + On Hit Damage - Armor Reduction
The attribute damage formula for Arcane Warriors would be Spellpower - 10 * [(Weapon Modifier) * (Style Modifier)]
If both Arcane Warriors above are using a sword such as Spellweaver (weapon modifier=1) the AW with 30 DEX will have a +5% chance to hit but will do 16 points less damage than the AW with more spellpower if he manages to hit.
I extrapolated the calculations for a level 25 Arcane Warrior with all points in magic from levels 15-25 and the attack values change to 76.2 for all magic and 81 for 30 natural dexterity. The difference in spellpower between the two Arcane Warriors remains at 16 so the attribute damage modifier doesn't change.
So if all you're doing is swinging a sword at something in front of you the argument boils down to this, would you rather have a roughly 5% greater chance to hit or an extra 16 points of melee damage?