It's splitting it up to put like-minded individuals into the same instance. If one wants to practice without exp, then they queue up for that. If one wants to practice with exp, they put in for that, etc.

It's like saying "We should have separates queues for cheques and cash." then saying "Wait, why not make one queue which allows people to use both?" which means longer queues as well as longer processing times as it is one person dealing with many, instead of 2 people dealing with many. Two heads are better than one, two queues are better than one.