With the way Discourse behaves, e-mail addresses are a unique identifier. By using the e-mail address attached to your account, you would be limited to a single forum account / email. This could even result in multiple EVE accounts logging into the same forum account.
If you check the CSM XI S2 minutes, you may see that we considered this approach for a while.
By attaching an artificial e-mail based off your character on the forums we have turned it around, allowing you a forum account for each character in your possession.
If there is enough backing behind this request, we can look into getting something done about it. Mostly just have concerns about maintaining the state between sessions/browsers.
Right now we plan no changes to this functionality.