With the sub account, if you accidentally post under the wrong user name, it's a simple click to change which account made the post. You get a drop down list at the very top of the reply box saying
Use account: - if you accidentally use the wrong one, just edit the post and use the drop down to change it.
Sub accounts do not show in your profile or on the member list. The only people who could see your sub accounts are the two admins (and it takes some digging which neither of us really have the time or inclination for, FWIW). Also, if at some point you want to merge all your posts into one account, you can do that with sub accounts.
Unfortunately, our software only allows one registration per email address.