Ravenlocke: I realize this post is a bit old, but I just got the IWD and BG games from GoG and was perusing the posts. I noticed a lot of dual-class and straight class characters for IWD1, which I thought was interesting. But I digress from my point. I remember making a pure mage, no specialty, and ended up making a tank out of him. With the right combination of defensive spells (I had four or five on him) mixed with offensive bang, I was letting my party hang back and let him mop up quite a few enemies. I'll have to share my party when I get it together. I think it would be interesting to play a variety of specialist mages though, seems like an intriguing concept.
Coelocanth: You'll have a real problem finding enough scrolls to keep them well stocked with spells of various levels. I recently completed a run with an Invoker, and Enchanter, and a Bard, and it was a challenge getting enough spells for them to have something in their spell books when they gained access to a new level. That's not to say it can't be done, as I did it, but I'd say it's optimal to have only two characters that use arcane magic in the party.
As for the 'melee mage' idea, it's definitely a viable solution. Once you get Tenser's Transformation, if you couple that with Mirror Image, Stoneskin, and Haste, you're a veritable one-man wrecking crew.
Good point about the scrolls. I think I used a pure mage, a cleric/ranger, a druid, a fighter and a fighter thief my first go-around, so I didn't have that problem. Worked really well. Also found that spiritual hammer is fairly powerful in the game too, which wasn't a spell I used often in the old forgotten realms games (PoR, CotAB, SotSB,,,etc..).
I wasn't clear on what I meant by a mage tank, my apologies. Cast... (trying to remember all the spells--its been a while) Stoneskin, haste, mirror image, minor (or major if it has it, forget which) globe of invulnerability. Maybe some cleric buffs too. Walk into horde of monsters, throw spells around, walk back out unharmed. The globe protects you from your own spells, so you can drop 3rd (minor invulnerability)-4th (major invulnerability) level AOE spells all around you without getting hurt and any higher level spell that's not AOE.