Why do you need to send a transaction out with NXT to secure your address?
NXT addresses are only 64 bits long. So to find a passphrase that matches it should only take on average 2^63 times.

But once you send a transaction from the address, the address gets permanently locked to the public key you used to make the transaction. The public key is 256 bits. So now even if someone cracks the address, they can't do anything at all until they crack the public key.