To be completely honest, I don't mind some of these kinds of requirements.
PERHAPS the big prblem of the crafting system is that it allowed us to make gear (very easily) that surpassed raid gear.
Holy Burst is a pretty powerful prefix - making it rare is no big deal IMO. I'm sure that's an unpopular view, but if I had to grind out a stack of Shrouds to make a LitII, then why shouldn't I have to grind out a stack of favour to make a holy burst GEOB weapon?
As to the premise that 'this means players would have to buy the adventure packs' --> well NO, they would not HAVE to buy them, they could content themselves with Holy instead of Holy Burst. The game is FTP, but it only remains a viable game if people spend money on it. You want Holy Burst, buy the packs.
Necro Packs are kinda underrated - the end quest of I, II, III is great for XP. And IV is a very good pack.
Oh - and I do NOT have silver flame favour on my crafter.