Originally Posted by qwerty3w
Very basic calculation:

hp = vitality
ma = magical armor
pa = physical armor
ma = magical damage
pd = physical damage

Suppose you have a balanced team 1:
2 hp 1 pa 0 ma 1 pd fighter
2 hp 1 pa 0 ma 1 pd fighter
2 hp 0 pa 1 ma 1 md mage
2 hp 0 pa 1 ma 1 md mage

Against a purely physical team 2:
2 hp 1 pa 0 ma 1 pd fighter
2 hp 1 pa 0 ma 1 pd fighter
2 hp 1 pa 0 ma 1 pd fighter
2 hp 1 pa 0 ma 1 pd fighter

Assuming simultaneous turns:
In the first turn, team 1 can deal 2 magical damage and 2 physical damage. In the best case, through focus fire it can take down 1 fighter and reduce another fighter's hp to 1.
Team 2 can deal 4 physical damage, it can take down 2 mages. So you end up with:

Team 1:
2 hp 1 pa 0 ma 1 pd fighter
2 hp 1 pa 0 ma 1 pd fighter

Team 2:
1 hp 0 pa 0 ma 1 pd fighter
2 hp 1 pa 0 ma 1 pd fighter
2 hp 1 pa 0 ma 1 pd fighter

Team 2 is clearly at a advantage.

Of course it is a very simplified model and does not take other factors into account, but it should help explain the problem a bit.

Like you say yourself your example is very simplified. Against a team of enemies with high physical armor and low/no magical armor Team 2 has a harder time than Team 1. Team 1 can CC/kill them quickly, whereas Team 2 needs to attack physical armor first before they can CC/kill.

Team 1 is more flexible, at the cost of taking longer to kill balanced armor enemies.
Team 2 is great against low physical armor, at the cost of taking longer to kill high physical armor / low magical armor enemies.

It's a tradeoff. To get back to the original question, this is the whole concept of the armor system imo. You can choose to do only physical damage or only magical damage with your team, or you can mix your damage types. All these choices have different advantages and disadvantages depending on the enemies you will face throughout the game.