I know exactly why I feel this way; it's a hangover from the "cursed" condition in MM3. I once ignored that, not realising that the party's luck was gradually dropping as time went on, until all of a sudden, my whole party dropped unconscious. And when the whole party goes like that, it's "game over". Ever since then, I've not let any adverse condition persist for too long, because I don't know what little diabolisms that the game creators have programmed in.

I believe extra resistances do make a difference, and IIRC according to Bones' study on combat they should do so. The problem is getting them high enough to make a big difference. The difficulty is that there are so many things you have to protect against that getting decent permanent protection is nigh on impossible. Coupled with that is the fact that the most effective items are those "of Protection", and the game doesn't hand out an awful lot of these out.

I regard the items which restore hit points and spell points very important, so I always collect some of those for each of my characters so that is two of the slots taken up, and the qualities of the artifacts and relics are built in, so those use up some more. There just isn't enough room left to fit in enough items to get your party's resistances up towards 200. What I normally do is get at least one character's dark magic skill up to 26, and if I then equip him/her with Guinevere and a Ring of Dark Magic, the spell "Day of Protection" gives a sizeable bonus. It's not perfect, but IMO it's the best compromise without hindering the characters' abilities in other ways.

Back to MM3 again - in that game, you could eventually get the resistances up to a point at which elemental attacks didn't hurt the party at all. The only chink in the armour was energy attacks; the only defence against those was a high-level "Power Shield" spell.