If I were a player and a DM gave NPCs one of these weapons and used it against us, I'd force the group to save them and retire my character and bring in a Psionic warrior with an exotic weapon prof....If the NPCs can do it so can the PCs....if not, you need a reason that can't be overcome by a feat or an spell or enhancement that automatically grants proficiency.
I'd be perfectly happy rolling around 11d8s as base damage after around 11th level. Eventually even more dice.
The DM can do plenty of things that the PCs can't copy...but deliberately using weapons from strange sources just to abuse dice progression is in that special place where PCs can say 'hrm...that sucked...our turn!'
I would expect most players to miss a dice progression abuse as a -hint- that the monsters were from some place else.
I have a player running a goliath barbarian with a large goliath greathammer. He does 3d6 +7 damage (he has a 20 STR). He hasn't quite learned how to most effectively use his rage and Power Attack though. I have another player who keeps wanted to use his domain spell Enlarge Person to increase his size. I don't want to think of the damge he would do then. . .