I believe in NXT checkpoints exist only to force users to upgrade when there are new core features added to the software (mandatory upgrades), as they require most of the forgers to upgrade to keep network consistent and with no forks. But that is only because NXT is being developed actively. Once most core features are implemented, no checkpoints would be required, that's my understanding of it.

There is no "nothing-at-stake" problem. No entity can ever buy 51% of the coins in existance, hence PoS system is protected immensely better than PoW system. The cost of attack on a PoS system, once it's out of its infancy and has a large number of nodes, is infinite. The more coins you buy to carry out this attack, the higher the price gets, before long everyone and their mother will be buying that coin driving the price thru the roof, and you'll exhaust your fiat resources long before you can gain anything like 51% of the coins.