Moldering shards are taken by oryx when you finish him. He uses up 20 for a better reward at a normal mode level. They weren't even implemented good then. On occasion you would lose 20 shards for nothing and gain 2 shards, basically negating what its supposed to give and taking your shards for nothing. You don't use shards for anything except when you kill oryx and even then they are next to useless now for hard mode.

I'm just saying they appeared to work as intended for me last night. I got the HM rocket launcher which is normally a WarPriest drop. This was in addition to a HM Scout and NM Hand Cannon I also got from the one kill. I agree, though that getting 2 shards as a possible reward for 20 shards is ridiculous. It's happened to me before. The other point about upgrading equipment with shards is a good idea.

I don't think rewards are specific for each boss or section. For Instance, i got 3 HM chest pieces 313 from war priest, 312 from golgoroth, and 311 from oryx (I swear this is true). I figured Warpriest only gave weapons, golgoroth gave chest and legs, daughters gave marks and oryx gave arms and helms but recently my drops have be different from this idea. We need more data to know where everything drops but in hard mode this does seem to be different.