A 51% attack on a Proof-of-Stake cryptocurrency is an unrealistic scenario and it's strange to bring this up as a critique against Spectrecoin.
Second that but it's not much unrealistic, in PoS coin if anyone wants to have 51% attack, they need to own more than 50% of the total supply. So, it's possible to have own more than 50% if the market capital of that coin is low.