Actually, I could point out that Cleric/Thief is useful in the Infinity Engine games for 2 reasons. First, Sanctuary, I believe, allows you to use certain thief skills without being broken (unlike Invisibility). Second, many thief skills are affected by Dexterity, and the spell Draw Upon Holy Might will increase it. (In fact, in Baldur's Gate 2, I believe every multiclass combination has some synergy that makes it useful.)

Also, I believe the rule is that you can backstab with any weapon a single class thief can use. This would include Clubs and Staves (but not the Staff of the Magi, which exists in BG2).

Of course, I have no idea how the Gold Box games implemented these rules. Maybe I need to use Unlimited Adventures to create some test scenarios. (It's nice being able to choose the amount of experience I start with; makes it easy if I want to test with level 40 characters, for example.)

There's no guarantee what you find will apply to other games, though; UA was made at the end of the series, so many things had been altered.

The thing is, it is a lot easier to test in UA than in other games. In any case, here are a few findings:

Destruction, cast by a level 40 Cleric:
Worked on a default Goblin (no surprise)
Failed on a Goblin with death immunity (no surprise)
Worked on a Goblin with 100% magic resistance (so high level Clerics can bypass spell resistance like Magic-Users can)

Resurrection, cast by the same Cleric:
Worked on a Halfling killed by Slay Living (with no CON loss)
Failed on an Elf killed by Slay Living
Failed on either character killed by Destruction (as if that spell weren't powerful enough already)

Did you check if the character was 'Dead' or 'Gone'? The only other way to be 'Gone' is a beholder's disintegrate ray or being level-drained past 1, as far as I know.

Destruction caused the "Gone" status, while Slay Living caused the "Dead" status, regardless of race.

Out of curiosity, is there any method in any of the Gold Box games that allows the "Gone" status to be removed? (Some of the early Wizardry games had ways to remove the "Lost" status, which is the Wizardry counterpart of the "Gone" status.)

Some tests with Disintegrate in Unlimited Adventures:
On a party member, Disintegrate causes "Dead" (not "Gone") and causes the loss of all items.
An enemy killed by Disintegrate will not drop any items.
An enemy killed by Flesh to Stone *will* drop items, so I recommend using it instead.

Is there any situation where Disintegrate will work when Flesh to Stone won't? Is it ever worth casting Disintegrate?

As a side note, my Magic-User who had something like 26/86 HP was unable to kill herself with Power Word, Kill.

You need to cast Cure Depression on your mage. Or just have her use her Dagger + 3 on her wrists.

Seriously, the Power Word Kill spell works against the maximum, not current HP (which isn't what the rulebooks say it should do). This makes it a lot less useful.

Disintegrate has twice the range of Flesh to Stone. Theoretically you shouldn't be able to loot a petrified enemy's stuff either, they having turned to stone with him, but that's the way the game works...

In Death Knights of Krynn Disintegrate would kill without destroying items (you were just Dead).

Also, the advantage of either Disintegrate or Flesh to Stone over, say, Slay Living is that they are not death effects and should work on undead.

According to the Pools of Darkness manual (haven't tested it ingame), it's the other way around.

If you are using Gold Box Companion I would suggest...

Human Paladin (tank1) ->Cleric @13
Half Elf Ranger/Cleric (tank 2)
Human Ranger (tank3) ->Mage @15 (3/4 way thru SoSB)
Elf Mage/Thief (more mobile than plate mages, can cast in leather +5)
Half Elf Cleric
Human Ranger -> Mage @9