"Nothing at stake" is in any case a chimera. It's a hypothetical beast that's never been seen in the real world. The idea makes some sense for currencies with a high block reward, but for a PoS currency that doesn't pay interest, like Nxt, the potential gains from forging on multiple forks are tiny, and not worth the loss of security. Remember that in PoS the forgers with the most to gain from shenanigans are also the ones with the biggest stake to lose if the currency does get broken. Upshot is that no-one forges Nxt on multiple forks today, and they likely never will.
Seems like similar logic to what most people agree is an acceptable amount of risk with Bitcoin(>%50 attacks).
With Bitcoin 51% attacks, there are also two stages. The first stage is to acquire 51% of the hashing power. The second stage is to use that power to attack. The difference is that Bitcoin has actually reached the first stage: there have been entities that could have attacked if they wanted to.
You're right in that the reason GHash.io didn't attack is that doing so would have destroyed their own business, and that's also the reason Nxt forgers don't forge on multiple chains.
So would you say that in general a Nothing at Stake attack would be significantly less likely than a >50% attack on a PoW network?
Has anyone ever seen any evidence of forgers forging on multiple chains on any network?
