I've seen this feature on a number of online conferencing systems. It allows people who don't do a lot of conferencing to confirm that they can do the rudimentary things like setting up their profiles, posting new messages, and replying to messages.
I have my e-mail address hidden, as a feature in "My Profile". I note that I can see my e-mail address on the "Reply to this Message" panel, but that it doesn't show up when it finally gets posted, for real.
When Flemming set up the first userid -- the admin id -- he used the primary e-mail address that I use. When I tried to register a userid just for posting messages, I noted that Phorum wouldn't let me do that. It maps to the specific e-mail address, and only allows one Phorum userid per e-mail address.
In this day where it's easy to get Hotmail or Yahoo accounts, this shouldn't be a big issue. Spouses who have chosen to share a mailbox can either share a single voice on this Phorum, or will have the get their own e-mail addresses.
What's the age of the youngest person you know who uses e-mail? As a matter of fact, I think that my youngest son had an e-mail address by the time he was 8 years old, but that's because e-mail comes as a feature with Instant Messaging registration. Most school-age children today don't actually use e-mail much. They much prefer the immediacy of Instant Messaging (IM).