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Landeril: The thing about a F/M/T is that you have to worry about the Arcane Failure Chance. :-/ which is why I suggested what I did.

Of course, you could do a Caster(Mage/Cleric) and a Combat Expert(Fighter/Thief)
mystral: Not really. Arcane Failure isn't implemented in BG 1 or 2 to begin with (equipping any armor just disables arcane spellcasting) but more importantly, in most fights you either don't need armor (against spellcasters) or you don't need to cast spells in the middle of a fight.

The concept behind a fighter/wizard multiclass isn't just to have an arcane spellcaster with higher hp and ac, it's that lots of arcane spells are self-only buffs that greatly improve your survivability (mirror image, stoneskin, protection from magic weapon, spell turning, etc..).

In other words, in many fights you should mostly play your fighter/wizard as a fighter that self-buffs before big fights to become pretty much invincible. A fighter/wizard is a far better tank in BG than a pure fighter, or any other class combination.
Later on, when wizard spells become better offensively, you can just sacrifice some AC (just buy the AC 3 bracers or something) for spellcasting, AC is not important if you kill everyone quickly and/or you can just make yourself immune to physical attacks.
Really? Its not. Thats kinda dumb. :-/ but then its the rule setting.
Arcane failure on heavy armour is an option in the tweak pack (at least for BG2 - probably for the BG1 version but I play on BGT) - I think it uses the p'n'p tables for % chance on different armour.

Still - better just to go with stoneskin et al
A fighter/mage can work as a fighter with various personal protective and combat augmenting spells or a mage who doesn't have to worry about wasting magic on trivial foes. Most often, however, they will be used as both, especially in a one or two-man group. Versatility is the key really.

The loss of armor is fairly trivial later on, as you can still use magical items that reduce AC, as well as shields.