Lowering fee opens up NXT blockchain to spam attack. Cost of a few cents to store 1K of data for 1 year (or permanently?) seems to be a pretty low price already

With NXT
the message is encrypted in the client but your passphrase unlocks it
it is unreadable anywhere except on a client that has the keys
the network of NXT nodes holds it so you can always get to it unless NXT ceases to exist.
no one can search, copy steal your ideas or communication.

You could also use arbitrary messages to communicate to yourself (sorry store) those secrets that you want to remember.