Dual classing is for high-level shenanigans, because XP costs to level up grow exponentially. So your Fighter 9 / Mage X is basically a Mage X with Fighter hit points and grandmastery in a weapon. (This is silly.) And whatever kit nonsense from Berserker or Kensai, obviously. (This is sillier.) At BG1 XP levels, though, it’s a total waste.
Alternatively, Fighter 2 / Whatever X is pretty strong. Two levels of Fighter HP, plus a bunch of weapon proficiencies/specialization, at basically no cost once you get to level 4 or so. This is also reasonable.
Multiclassing is for bosses who want to be the boss of everything, because a Fighter 9 / Thief 11 is like a million times more awesome than a Fighter 12. Similarly a F3/T4 at BG levels, or what have you.
Yes, indeed, Fighter/X is generally the way to go. Mage/Thief is very tricksy, true, but really isn’t much more interesting or powerful than straight Wizard.
True heroes are Fighter/Mage/Thieves, because being invisible basically at will while backstabbing with quarterstaves for 60+ (at low levels) to 200+ (at boss levels) damage is pretty strong.