Coin age is at stake.
Observe case in which cartel using modified client agree to mine a chain in secret alongside the main chain, and to double spend at an agreed time.
For this cartel, there are almost no consequences for a double spend. In the best case, they get their block reward and can double spend freely. In the worst case, they perhaps miss their miniscule stake reward which is set at 1% a year, but still get to keep their accumulated coin age. In the case of BlackCoin, this is even more severe as there is no upper limit to the influence of coinage on the ability to mine a block (whereas PeerCoin sets it to 90 days).
It's a case now of whoever gets to do the first doublespend on the BlackCoin network basically wins all the BTC in the buy orders, and the rest of everyone will be left being bagholders.
Bizarrely,
rat4 set about breaking MintCoin which improperly implemented low reward PoW on top of PeerCoin PoS protocol while not bothering to address or fix any of the problems inherent in his own protocol.
Sunny King "fixed" this with centralized checkpointing.
BlackCoin has no real value above PeerCoin, and probably less of a value than even nxt... to be fair it probably made rat4 and friends a fat paycheck though.
