At times getting your heroes to gather in the tavern to get a party make up you want can take a little timing.
If you split attack flags on opposite parts of the maps you might sometimes have all your clerics on one mob/den and if the other heroes don't have lots of pots and are attacked by something nasty or on a strong mob den(mobs come out when they destroy it) they are toast.
So I would argue parties can cut down on deaths, less ressurecting.
As someone who enjoys using wizards, having them with a group for a tank and healer really helps.
If you pointer over mobs you see they often have different strengths of their defenses to magic vs ranged and melee. A balanced party is mildy helpful here. Some of the abilities such as stun, iceblock etc also balance well off each other; a bit more useful. (ex-Mage freezes werewolf give healer time to top off tank.)
If I end up with an odd party of three it is usually 3 rogues. 3 rogues on one strong mob might kill it before the mob kills the lead rogue. Now later I might have a healer for them if I do a second clerics guild or even add a dwarf in the mix as lead or just to grab and tank as he will live a lot longer.
I would note the lead heroes speed is a good thing to keep in mind as a ranger or elf can run well ahead of the party they don't wait up to make sure there healer is near. (As long as they live til the tank and healer catch up you are fine.)
The other key factor is the attack/defend/explore behavior or the lead. If I have a party with a dwarf and a fighter, the fighter is usually lead as they seem more willing to attack especially as they get higher experience levels.
For demons advisor and final Majesty 2 mission, 2 or 3 parties of:
Paladin,DPS,Priestess of Agrela,Cleric are wonderful. A dwarf is a not too distant second. Two healers on a tank isn't generally needed but any dps stragglers not in parties or in other groups get the benefit of being near where the tanks that are really strong against the boss mobs.
It seems like there may be some preference of healing order by party. (IE heal my party first then neighbors) I am basing this largely off who is most likely to die when 3-6 parties are attacking and end boss. Think of when your first party or two gets to something nasty while the others parties are still running there. I have had times where none of my wizards died but my rangers or rogues died a lot, when they were ungrouped or with no healer etc.
I would be curious as to which combinations/use/non-use tend to reduce overhealing, increase overall efficiency, but that level of knowledge isn't really necessary.
Rarely out of experimentation or trying diffierent strats I will break and reform parties to mess with their make up.
Can you beat the game without parties, sure, but I find using some parties can be helpful and tend to speed up progression of the mission. Just another aspect of the game to try different things with.
Just my 3c.