The Duty Finder already tries to match one melee with one ranged when queuing for 4 man content. I'm sure it probably tries to match 2 melee and 2 ranged for 8 man. However to avoid extremely long wait times it also probably has a if statement that says "If X amount of time has passed match melee with melee or ranged with ranged." With 3 - 6 slots dedicated to DPS their wait time might be a little bit faster since the game is pulling more DPS into one place that it was before, but if people are not playing tanks or healers then those 3 DPS are still going to wait till the DF can give them to the DPS.