Sounds like it already does kind of do that. From StavrosK above:
"It maps multiple emails to the same identity, in that you use the same password to log in with all of them. The end site gets the email you log in to the site with (I can log in to Persona with email A but let all sites know email B every time."
So based on that, if you want to expose three different identities to 3 different sites, you can set up Persona to do that automatically, without having to remember the separate emails and passwords you used for each one. I guess the only problem would be if you needed multiple separate logins to the same site, but unless it's your own site, presumably that should be rare.
"It maps multiple emails to the same identity, in that you use the same password to log in with all of them. The end site gets the email you log in to the site with (I can log in to Persona with email A but let all sites know email B every time."
So based on that, if you want to expose three different identities to 3 different sites, you can set up Persona to do that automatically, without having to remember the separate emails and passwords you used for each one. I guess the only problem would be if you needed multiple separate logins to the same site, but unless it's your own site, presumably that should be rare.