Let's say this game was based on ranking. So you have either bronze, silver and gold ranks. So those ranks are spread on:
How much damage you do
Healing on time
Not over healing too much.
DPS actually use a rotatio and do damage as they should do, hence dps.
Tanks using cooldowns proppely, holding aggro
Healers dpsing
Tanks doing a decent amount of damage.
Now let's pretend you are very good so you end up with silver and gold rank players, but if you are bad you end up with only bronze players. Which means people who do blizzard spam etc in dungeons, healers clicking every 10 seconds. Yes even farm groups will end up you being with the same skilled people, so you have 5 wipes a row in farms then maybe 1-2 kill between. Would this be unfair for you all? Or would it be fair?
Basically you have a huge chance of ending up with same minded players, so if you don't aoe in dungeons, you will meet the same type of players in dungeons.