Basically the Bloodborn and Ancient Bonds orbs provide a level of certainty. You are certain to get all the cards after a set amount of orbs. Since you can’t buy those after buying the set amount of orbs it means that any cards that you disenchant from those sets (if it were possible) would be forever gone. There could be a chance in the future where you’ll want to use those cards from factions you don’t use. If you could and did disenchant those then a few months (or whatever time later) you wanted those cards, you basically lost them forever.
The difference between Bloodborn + Ancient Bonds compared to Shim’zar and the Core set is the certainty of getting every card from the set. It may seem expensive at 300g per orb but you are getting 3 copies of each card in the orb. Whereas, Shim’zar and the Core set you are not certain to get every card from the orbs and you lose value from the cards when you disenchant them. You could say that at least the Spirit goes towards cards you want but you’ll still have to grind for Spirit to be able to craft the cards you want each time you fail to get the card you want from the orb. Also the Bloodborn and Ancient Bonds business model have been shown to be favourable by the community in a forum poll some time after the Bloodborn, where the majority of the players liked it.

