Ok , so if we buff up DM with a 1,5 multiplier , why not EVERYTHING ?
No.
The edge of THF is more DPS on crowd , less feats .The edges of TWF is more DPS on single target , more sinergy from buffs/bonus .But it cost twice the grind and a lot of feats that you MUST have , unlike THF that works well just with PA and cleave , and the game doesnt allow the same class of weapon : nothing like a eSoS for TWF .
Each has its own strengh.
So enjoy the attention turbine gives to you ( like the creation of a THF feat line or the high requirements of DM , coz few TWF can afford to have such a high charisma ....).
And no , 1 minutes is already very nice , no extended duration needed and no perma DM.
Why ? a perma damage boosted melee is called a fighter or a barbarian.It cost them their whole toon.Not just a couple of points.