With current blockchain and network capacity then yes spamming is a huge issue. But I doubt lowering fee will cause it unless someone plan to attack the network. I send my messages free via whatsapp, skype, yahoo mess, snap chat and a bunch of other apps. You can't compete by lowering the price with NXT messenger. It has to be a special type of message in the blockchain.
When the novelty wears off, no one is going to pay a dime to send a conversational message via NXT.
Secure communications seems like a good selling point....as non of the above APP's do that.
NOTE: This is an already released feature the genie is out of the bottle.....i;m just arguing to lower the msg fee....thats all.
EDIT: That would be foolish to suggest Nxt could compete with whatsapp....but there are a lot of people in this world who would use Nxt for secure communications.