Nxt doesn't use checkpoints. Although it adds features fairly often, its core model doesn't change often and new features don't make it harder to attack. Even if they did, I don't see how that would affect mounting a 51% attack. Getting to control 51% of Nxt would be implausibly difficult because of the difficulty in acquiring that many coins.
So you have already forgotten that one time when a hacker stole 5% of all the coins, and with that, 5% of all the hashing power?
A hack of an exchange is not a 51% attack. 5% is not enough coin to mount an attack. The hack had no effect on block-chain security; the hacker couldn't do anything that Bter couldn't have done.
And I don't see what's so "implausible" about buying half of the coins when the market cap is so small.
If you try to buy so many, the price will rise. If you succeed, you will find you are attacking yourself.
Did you know that you don't even need to own the coins to perform the attack? You just need to have owned them at some point, and begin your forks there.
You have to have owned them within the last 12 hours or so, because although Nxt doesn't have checkpoints, it does disallow block-chain reorganisations past 720 blocks. This means to do what you suggest you have to acquire 51% of Nxt and then sell them all off within 12 hours. Dumping in such volume would crash the price, and you'd lose a lot of money.
With PoW at least the attacker will have trouble recovering the costs of his mining gear which will become obsolete.
The mining gear can be used for other PoW coins that use the same ASICs.
See what Gregory Maxwell (nullc) thinks about PoS:
He's talking about Peercoin. It's a different algorithm. His comments don't apply to Nxt.
No, he's talking about PoS too. If you have any doubts go ask him yourself. He's always in the #bitcoin-wizards IRC channel. PoS is not considered a viable alternative to PoW yet. There will have to be another breakthrough to make it workable.