I's recommend having fighter, cleric, non-specialized mage, sorcerer (for IWD 2, for IWD 1 - specialized mage), rogue. That basically leaves 1 slot (2 slots if you want to take 1 mage only) for whatever you want. I would strongly advice against taking paladin as they end up quite subnediocre - not good as fighter, not good as cleric.
I wouldn't say so. E.g., in Icewind Dale I, difference in benefits between 17 Str and 18/00 for fighters is more than triple. Less so for other stats, but still, difference is quite big. And low levels in AD&D with non-optimized stats is a torture. For IWD I I'd recommend having Constitution, Dexterity and primary class stat at 18. For IWD II, I didn't play it much, so can't advice anything, but I think Darvin explained it for IWD II quite well..

Straight from the manual:

IWD1: Strength Effects

16 - +0 THACO / +1 DMG
17 - +1 THACO/ +1 DMG
18 - +1 THACO/ +2 DMG
18/01-50 - +1 THACO/ +3 DMG
18/51-75 - +2 THACO/ +3 DMG
18/76-90 - +2 THACO/ +4 DMG
18/91-99 - +2 THACO/ +5 DMG
18/00 - +3 THACO/ +6 DMG

As you can see the difference between 17 and 18/00 is IMMENSE and much more then 3x more effective but it is by no means necessary to make a character with 18/00 as there are far easier ways to attain this without power gaming. Early game you can use a 2nd lvl strength spellslot on a mage and it gives a long lasting buff that can boost you up closer to 18/00 and eventually you will find gloves of ogre might. Personally 18/00 feels way oped and I don't think any non power gaming build should have it.

IMO bards are a waste of a slot. They fight no better than a thief, they gain spells more slowly than even a multi-class mage, and their lore ability is useless due to the Identify spell.

I haven't played IWD2 yet, but the 3e rules have much more linear scaling of effects from stats: each stat is given a "modifier" (see below) according to what level it is, and the vast majority of bonuses/penalties applied will be that modifier. If you have 18 strength, for instance, you will have a +4 attack bonus (equivalent to THAC0) and a +4 damage bonus. With 16, you'll have +3 for both of those.

The modifiers are as follows:

1 -6
2 -3 -4
4 -5 -3
6 -7 -2
8 -9 -1
10-11 +0
12-13 +1
14-15 +2
16-17 +3
18-19 +4
... ...

As you can see, it's much more linear: dropping the amount a statistic is away from the average person's (10, by the rules) has the effect that you would expect. One thing to mention is that if you don't plan to improve a statistic further then it's probably not worth raising it from an even to an odd number, as that'll have little to no effect.

Disclaimer: I'm speaking from my knowledge of 3.5 and pathfinder rules here, which are slightly different. I don't think they'd change anything as drastic as the modifiers system, but I may be wrong. It's also possible that 3.0 doesn't let you increase your statistics on some level ups like 3.5/pf, in which case you may never be able to increase your statistics after character creation. Also, stat generation may use a different method in IWD2 to the BG games due to the difference in ruleset, invalidating most of what I said.

Bards lvl 11 spell gives party wide -2 AC, +2/round heal. That's far from useless! One could argue that a fighter would be more beneficial but that depends on party make-up and what you want to do. I personally love having a specialist mage and a bard to cover the missed spells. I assume you are talking about IWD1 because you mentioned multiclass :D

The Lore ability allows you to identify without wasting a spell choice on Identify for spontaneous casters or without wasting a daily slot for Wizards, and you can identify in the field without having to travel to a merchant (and saves you the cost).

You can make that argument about a number of the classes and races. If you have a Sorcerer, why take a Wizard? Just add another Sorcerer. If you have a Fighter why take a Barbarian? Or vice versa.

Even if one accepts the argument that Bards are a waste (I don't), they're fun to play. And this is coming from a self-admitted powergamer.

*edit* If you're talking about IWD1, then they're even better.

Bards are not only not useless, but get ridiculously overpowered as they gain new songs (around lvl 9-11) due to the Siren's Yearning and War Chant of Sith, and their Lore saves a lot of time and money. In fact, I avoid playing a bard these days because they make the game too easy.
I once installed the IWDfication mod for BG2 which let me use the bard songs and it completely broke the game. They might not be that game-breaking in IWD but they do lower the difficulty by a significant amount.

That and bards are kind of a silly class to play. I mean, your primary contribution is to go into battle and *sing* at people; meanwhile, you have five other characters trying to pretend that this is some high stakes story that they are taking at least halfway seriously, and I guarantee you after a week of fighting while Elan over here is crooning to the strumming of his lute, even the party Paladin is going to start dreaming about strangling the little twerp. I've had people try to come up with ideas for how to make bards interesting, but I have yet to see it actually pulled off in a game.

My problem with bards is while they are singing they can only move and do nothing else..I am right on that aren't I?
If you're trying to play the game with a 4 person party you can't afford to have a guy just standing around singing.
If you're going through the game with 6 people then I can see how making a bard would be ok. I hate having to manage 6 characters so I try to go through the game with 4 characters only.

Thats true about a bard but mages have fantastic buff spells and if you use a bard that frees up pre fighting buffs for your mage to memorize something else, ontop of that, warsong of the sith gives -2 party wide AC buff that stacks with everything and 2hp/round heal. Not shabby. Really it's up to preference. Bards suck with about anyweapon anyways so singing is their best bet in combat but its not all that bad.