Someone already asked me this question for the V0.0.1 of this project. In fact, we need to use the private memo key to be absolutely certain that your recipient is who he claims he is.
If we do this with another encryption system, anybody could create an account with the name of a powerful account, and start to scam people. This problem already happened on other mediums like Steem.chat and Discord.
interesting point of view. thx