CoD matchmaking isn't trying to fill specific roles it just matches people based on rank etc. It's a lot simpler dividing 100 people into say teams of 4 random people than 100 people into teams of 1tank 1 healer 2 DPS. 100 people select a CoD map and it's easy to just divide them up and go. Not so easy for an FFXIV instance. 100 people is 25 groups but there may only be 10 tanks queued so 60 people (20 more partial groups) are waiting for a tank. What are the odds 20 straight tanks queue up? The Xbox system wouldn't change this.