Aren't most queue times long because tanks and healer are in short supply?

How would adding another DPS slot help? You would then just have one more DPS waiting on a tank/healer...

Yes it would take them out of the queue, but without the others, they are still waiting, which is the issue your trying to fix...

Tanking and healing need to be more interesting to get people to queue with them more, then you will see a larger drop in DPS queue times, like you usually do when new tanks and healers are introduced for the first few days of a expansions or jobs release.