With the present amount of information relased by Apple the only known advantage is that you are declaring yourself as a user of "icloud" any time you send an outgoing email. This may or may not be considered an "advantage" by yourself. It is clearly a "benefit" to Apple as is offeres a degree of advertising that you are a user of their service.
Will there be advantages in the future... only Apple knows!
Apple has made no provisions for anyone with "@mac.com," "@me.com" Apple Ids to change them to @icloud
Incoming email is accepted if addressed to "@mac.com," "@me.com" or "@icloud.com."