[color:"orange"]that quality shouldn't affect me[/color]

Those damage types do affect various opponents differently, though, so it makes sense to have a resistance value to describe the relative vulnerability to each elemental attack (it has to be there in some form, though it could be partly hidden for your character). In principle you could have these spells doing physical damage, but with a damage bonus (or penalty) based on the type of opponent. Either way you would effectively have the same situation, but using resistances is easier than adding an additional combat subsystem for certain types of spells.

Also, though this is more of a conceptual problem, elemental damage may not exactly correspond to physical damage for all types of armour. Full plate offers better protection than leather for crushing, slashing and piercing damage, but if a water spell floods an area deeper than the character's height, or turns the ground to mud, I'd much rather be wearing the leather. A silk robe would be much better protection against a cold wind than a cotton robe, but they may offer similar protection against physical damage types. It is unlikely the combat system will get this intricate, though, since these are relatively minor influences, for the most part.

Earth spells could just do crushing damage; pretty much anything resistant or vulnerable to a rock will be affected similarly by any blunt weapon. However, a water based creature may be much more affected by sand or dirt than other creatures with similar physical resistances. If earth damage were to be eliminated (without adding damage modifiers based on the opponent), there would need to be restrictions on the types of earth spells available or on the creatures in the game, so that damage to all opponents was reasonable and made sense.

[color:"orange"]Requiring protection against a shaman's raindance just seems queer to me.[/color]

Not if the water is ice cold, or it is acid rain. <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />