I´d say REALISM FIRST, you can indeed model many functions with only one number with modifiers, so WHAT NUMBERS SHOULD BE PORTRAYED, I think PANZER GENERAL had good values, it had different values for different kinds of combat, for close combat you had CLOSE ATTACK, that would do for all melee units, since melee combat IS close combat, perhaps a single defence number would do for all defence?
So:
CLOSE COMBAT (ALL UNITS HAVE THIS IN CASE OF MELEE, Melee units, mounted units with swords and spears etc.)
MISSILE ATTACK (archers and all pre-gunpowder missile units)
FIRE ATTACK (musketeers, riflemen, machine guns, modern infantry, tanks against infantry etc.)
Fire attack and missile attack could be grouped together under the value RANGED ATTACK
HE (high explosive) ATTACK (tanks, cannon, howitzers, bombers and fighters against ground units etc.) Could also be just FIRE ATTACK, but more realistic if Explosives are placed in a class of their own with their own value.
HARD ATTACK (tanks against tanks or anything against tanks, swordsmen or greek phalanxes have VERY low Hard attack values

Basically anything attacking a "hard target", like a tank) Could also be just FIRE ATTACK to save space and values, anti-tank guns also use gunpowder, so it could be just FIRE ATTACK.
SEA ATTACK (for sea units) Sea units could also use Close combat (boarding actions by older ships) and FIRE ATTACK (cannons, missiles etc.)
AIR ATTACK (anything against air units, pre-gunpowder units would have an air attack of 0, you can´t really throw spears or shoot arrows against an F-22 now can you?) This would cover GROUND-TO-AIR fire like anti-air missiles and AIR-TO-AIR combat like fighters against each other so this would be necessary or perhaps it could be done with a modifier to FIRE ATTACK, all ground units would have a -100% or -50% value against air units and air units against air units still use gunpowder driven weapons or explosives, so it would be FIRE ATTACK AGAINST AIR UNITS, so basically RANGED ATTACK OR FIRE ATTACK could be used instead of AIR ATTACK, ground units would simply be poor at shooting aircraft (a modifier to fire attack), napalm bombs would simply be RANGED or FIRE attack with a +100% bonus against infantry.
So, three or four attack values CLOSE COMBAT, MISSILE ATTACK (arrows) AND FIRE ATTACK (gunpowder and explosives), perhaps HARD ATTACK against tanks would cover all weapon classes from antiquity to modern weapons.
Alternatively there could be just CLOSE ATTACK (melee) and RANGED ATTACK (all forms of ranged attacks from arrows to missiles) and HARD ATTACK (this because TANKS are INVULNERABLE to bullet fire of let´s say riflemen or musketmen, bullets just don´t touch tanks, so anti-tank weapons have to be in their own league and modern infantry can have several weapons, they can have bullet weapons against infantry and anti-tank rockets against tanks)
Can all this be done with a single value? Modern infantry and tanks have modifiers to a single value, tanks have +100% when attacking infantry (tanks just overrun infantry) and infantry has +100% modifier when defending against tanks after the development of rocketry and anti-tank rockets promotion.
For defence there could be CLOSE DEFENCE (melee defence, close combat skill, martial arts etc.) MISSILE DEFENCE (agaisnt arrows and bullets alike) HARD DEFENCE (tanks and battleships) Tanks and battleships are impervious to bullets but modern infantry carries anti-tank rockets, so there should really be two attack values FIRE ATTACK for bullets and HARD ATTACK for bazookas and anti-tank rockets, modern infantry would use FIRE ATTACK against infantry, cavalry and all "soft" targets, like flesh and HARD ATTACK against TANKS and other "hard" targets.
In their own age knights and armoured cavalry could be considered "hard targets", very hard to kill, unless you got really really close and slit their throats, which of course was more easily said than done.
So, four attack values CLOSE, MISSILE, FIRE (missile and fire attack could be unified under one value, RANGED ATTACK) and HARD attack and THREE DEFENCE VALUES ClOSE or MELEE DEFENCE, MISSILE (bullets and arrows) and HARD DEFENCE (Tanks, battleships, battlemechs, mech infantry etc. etc.)
Or maybe two defence values, CLOSE DEFENCE (against melee and anti-tank rockets are close range weapons of modern era) and RANGED DEFENCE (against bullets, arrows and missiles)
Example: a knight has metal armour so he´s almost impervious to melee attacks you can´t really hack your way through a metal plate with a sword, BUT you can kill the knight with a RANGED ATTACK using a crossbow, so high close defence value, but low enough of ranged defence for a crossbow bolt to pass through, not to mention bullets.
Again could be done with a single value using modifiers, a knight has +100 defence against melee, but crossbowmen have +100 attack against armoured cavalry unitclass.
I advocate multiple values because it gives you more ground to work on with bonuses and promotions.

You can have CLOSE ATTACK bonuses like martial arts or better swords, HARD DEFENCE bonuses like ERA (explosive reactive armour) armour or Chobham armour making tanks tougher against anti-tank missiles of infantry and helicopters etc.
Multiple values also gives you different new aspects like in Panzer General, Tanks are vulnerable to CLOSE ATTACK so multiple infantry units can besiege a tank unit and finish it off with close attacks, of course, this might only be doable if the tank is in a forest where it can´t see anything or manouever or IF THE TANK RUNS OUT OF AMMO, this too could be modelled into the game, SUPPLY, you can resupply your unit any turn, but it consumes the move of the unit, if you´re resupplying you can´t attack that turn and if a unit is encircled, surrounded, it can´t resupply, so you can destroy an enemy army by encircling it and starving it
So, Attack and Defence values, plus some more like SKILL (how good swordsmen the unit are or how accurate shots) Could also be just bonuses to attack values.
MORALE, how easily the unit retreats or runs away, THIS IS CRITICAL, in earlier days combat was often decided by a units morale, he who´s nerves held stood their ground and won while others lost their nerve and fled the ground. Would you allow someone to charge over to you and stick a bayonet into your gut or would you run away?
MORALE and ORGANIZATION would be used in conjunction with STRENGTH, when a unit suffers casualties in combat, it loses morale and gets disorganized, when morale drops low enough or runs out the unit will flee and if the unit gets completely disorganized, it is just a mass of men easily butchered by a more organized enemy, that would be the function of those two values.
With multiple values you can do different things, that´s mainly why I´m advocating it.
For example: With CLOSE ATTACK value you can have all the melee units use mainly this value, but since they do not have FIRE ATTACK or RANGED ATTACK they don´t shoot first before melee, like archers, this is represented by the "first strike" feature in Civ 4. Since swordsmen don´t have any HARD ATTACK, they have a snowballs chance in hell against tanks, though this can be done with a single value by making all melee units have -100% against tracked units or armour units.
Example: a fight between a swordsman and modern infantry, the infantry is OUT OF AMMO (SUPPLY would definetely get my vote, model SUPPLY into the game I say) and doesn´t get its first strikes, so it doesn´t kill the swordsman at range, after that the melee begins, which one will win? A rifleman with a bayonet or a swordsman with a sword, I´d say swordsman wins because they are highly trained in melee, whereas the rifleman or infantryman is just trained to shoot and a rifle versus sword in a melee is pretty much going in the favor of the sword, I´m guessing you have all seen "The last samurai"? :-D
So, the question boils down to this: what tricks can you do with multiple values, that you CANNOT do with one value?
What different tricks does a given value give, what can you do with a value?
Is a value REALISTIC, is it HISTORICAL?
Cheers!