The chances of getting anything worth keeping are so slim they could just offer 6 free a day forever.
That's... not really true though, about the slim chance of getting anything worthwhile. Despite my joking about Oseem (thanks for that shield block rate on staff, bro), I do manage to get at least one keeper on one of my two characters on most monthly campaigns. It's really not supposed to be a targeted thing, so I don't mind one truly random occasional campaign that usually gives garbage but occasionally provides something amazing.
My main has TH+2 pieces allowing me to get TH4 on all jobs, some really good/BiS WSD+8~9% pieces, some killer MAB gear. I have a mage alt/mule with 5/5 Refresh+2 Chironic (and 3/5 Refresh+2 Merlinic), a couple very good MAB/Burst pieces, some very good random RDM melee/WS gear.
That being said, yeah, I don't think it would really destroy balance if they made 6 a day DM chances a permanent thing.
Or, maybe instead of requiring Dark Matter, something like consuming one stone of EACH of Fern/Pellucid/Taupe for a "Techniques" chance would be fair.