I consider a balanced party to be one that contains at least the following:
1 character capable of casting Mage spells (for Enchanted Blade and Missile Shield)
Either:
* 1 character capable of casting Priest spells (for Armorplate, Magic Screen and Heal All)
* Both a Bard and a Gadgeteer (you can get access to those 3 spells this way)
1 character who can cast Identify
1 character who can cast Heal Wounds (because Heal All isn't available for a while)
1 character with the Locks & Traps skill

If you omit one of these characters, the game will be a lot harder than it would be otherwise. Of those roles, Locks & Traps is probably the easiest to do without, especially if you have multiple characters capable of casting Knock Knock. Note that some characters can satisfy multiple roles (Bishop can do everything above except Locks & Traps).

For Wizardry 6 and 7, the requirements are probably similar, except that Identify isn't needed in 6 due to the lack of unidentified items. Note that the Bard/Gadgeteer combo can't replace the Priest here (especially since 6 and 7 don't even *have* Gadgeteers). On the other hand, note that Bards get mage spells in 6 and 7, but not 8.

For Wizardry 1-5 (4 excluded), a balanced party has both Mage and Priest spells available as well as a Thief (with 16+ Agility) and a Bishop (solely for item identification). You also need characters who can wear heavy armor to cover the front line.

Wizardry 4 is completely different, to the point where none of this discussion can be applied.

Hmmm, I have never found bards or gadgeteers to be particulary useful (and certainly not mandatory). They are like worse version of spellcasters but using stamina instead. On the other hand bishops are way too good - they can replace any spellcaster and do everything by themselves saving space in party. So, there is no need for balanced party, because there is no balance between classes.

The thing is, if you don't have any character capable of casting Priest spells, the combination of a Bard, Gadgeteer, and a character with Psionic spells is good enough to replace it. Bards get Magic Screen and Gadgeteers get Armorplate. At level 9 (I believe) and with enough skill, you can just repeatedly Shift-U the item until the spell is cast at Power Level 7. The Psionic covers Heal Wounds (important before you get access to Heal All items) and Identify Item (necessary because, for whatever reason, Identify scrolls are only power 4).

Also, don't forget that Bards and Gadgeteers get exclusive buyable +20 Strength accessories, and Bards additionally get a whole set of unique equipment that provides nice stat and skill bonuses. Furthermore, they are the only classes that get Locks & Traps but not Stealth (I dislike the way Stealth works, so I avoid it), and Bards get a nice bonus to Communication, allowing you to get better shop prices.

That's right, but you don't need bard + gadeteer and their toys because of bishop who covers 3 shools of magick (all 4 if you tryhard). They are just redundant and minor bonuses like class exclusive items, better resting or triple shot crossbows cannot compensate for that.

PS. another thing - omnigun causes blind which makes mobs run away and is super annoying.

I would add at least 1 front line fighter type to stand between the bad guys and your bishop... fighter, lord or val are best for this roll.

sorry but ime a bishop does not = bard in the game until much later... I like both bard and bishop but they are different and a bard casting shrill sound will be stronger than any spell a bishop can cast at that point in the game. for that matter a gadgeteer with lightning rod will out damage the bishop as well.

I agree with Hrymr that they are optional (not required) but its your game guys.

I'd definitely agree on the priest spells. Although I'd hesitate to name it "balanced" it will be a lot more difficult otherwise. Locks and traps, yes: that makes for a balanced party that doesn't lack the basics. It's not too hard to replace but a lot more comfortable if you have it.

As far as Bard & Gadgeteer go I don't know about that... It is comfortable but really not necessary.

More importantly though: you miss out on the most crucial aspect imo. One should really have one strong melee front character, typically a Fighter. Melee damage cannot be replaced. Also a tank makes things a lot more comfortable and easier. He is also the main recipient of the priest spells.

I just played through with a Mage wielding a Staff of Doom so I am well aware of that. However: if you accept some half-baked frontline you certainly can't call Bard and Gadgeteer a necessity, can you? *As far as balance is concerned it sure appears a lot more balanced to me to have casters, at least some Divinity, as well as a sturdy, hard hitting front.
You are right about the Priest spells. It is not the tank that needs them too much. If it is a Fighter, which is what I thought of, you want to buff him. So Superman is a good thing at least. Ideally you'd want it to turn out the way that your front takes all the damage so you'd only heal those, which is pretty effective if it works. Alas it is true: you'll likely end up saving your weaker backline a lot more often.