Only the two first players in the party can engage in melee combat, so if your Mage or Priest charms or holds monsters that have only melee attacks, then those monsters cannot fight unless the Mage or Priest is at the front (which is unlikely). However, if they charm or hold monsters that have ranged attacks via spells, then those monsters will indeed attack using spells (and your character will get credit for any damage they do). So generally, my strategy is to have my Priests wait for spell casting monsters before they use their (quite limited) number of hold spells. Note, however, that your characters do get a chance to make level when the monster is charmed or held (same as if you kill them), so there is some value is charming non-spell throwing monsters.
There is one other thing I might mention. Mages have summon spells which can summon a charmed companion. Since they do not get any direct benefit from the summon, if the monster cannot fight (which will be the case if the monster is melee only and the Mage is not at the front), then the spell is pretty much useless. Because of this, IF the monster summoned is a melee only monster, then the monster will be attached to one of the first two players in the party (so they can fight). However, any damage the monster does will still be credited to the summoning mage, and not the fighter.
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