So... You wanted loot from a duke with just the trappings sets and didn't have a medallion? Is that what you're saying here?
Those 6 sets would net you two medallions and use them up... That is if you had the shards for them as well. That is why he stated those sets were useless for summoning a Duke and you not getting loot.
While he could have been nicer, I am sure he was bombarded with whispers and offers, so it can be forgiven in some circumstances.
http://db.vanillagaming.org/?quest=8332 Each lesser stone elemental gives one crest.... So your math must be wrong thinking 6 sets nets 3 dukes and some change.