Just queue up as DPS. At least that way you can have some fun and do something still. If loot is on your mind there's no reason to worry either, you can pretty much win everything in ST with greed rolls as most players still just wait for the oils anyway.

There's very little need for an additional 3 tank slots. The only possible reason I could imagine it being helpful is in it increasing the odds of one of the tanks actually being willing to take initiative and tank. Three tanks standing idle and twiddling their thumbs is one thing, but most certainly 6 can't do the same right?