Some enemies have low to 0 physical armour, some have low to 0 magical armour. Focusing on only 1 is kinda dumb when you can break 1 for free. This idea completely nullifies that and makes everyone an armoured tank unless they lack both. What's more, different armour types block different cc affects, so only relying on one damage type can really hamper your options.
But it doesn't hamper your options at all, in fact, it drastically increases your damage output in the vast majority of scenarios. Unlike a split party which has to individually focus each enemy based on their own strengths and weaknesses, an all-same damage party can just focus the biggest threat and be done with them.
