I'd like to see such a detailed article about PoS and PoW+PoS combinations.
Here is a whitepaper which explains the nothing at stake attack.
http://download.wpsoftware.net/bitcoin/pos.pdfin my view this is much worse than a 51% attack because it requires less processing power and once someone has successfully carried out an attack their success rate increases exponentially. (unlike PoW attacks which decrease after each successive attempt, because the network/community is able to react to an attack)
Pow+Pos combinations in their various flavours are vulnerable to both 51% attack and nothing at stake attack at different times. Anyone who tells you their coin is
more secure without providing proof is
lying.
the problem of securing a PoS coin is not easy and that is why there are so many all-star dev teams backed by multi million dollar funding working on it. (bitshares, etherium, NXT, ripple etc etc etc)
if you think that Karma has the necessary expertise and funding to pull off a secure PoS or even Hybrid implementation
without issue then by all means go right ahead. But if not then you are just asking for the community to take a gamble that they might get a short term reward at the risk of seriously devaluing their investment.
here is some further reading on some of the more popular PoS systems and why this author thinks they are flawed.
http://bitshares.org/delegated-proof-of-stake/