It's not a simple matter. They give it thought and decided, as other games have, that showing the account name was a better option due to abusers. (Of course, if they can figure out passwords or secret questions, then that would be a true problem.)
But, in the guild window and the likes, we should see character names also.
Using an email with two-factor authentication thwarts this and most free emails offer this now anyway.
Seems like all Mega server type games use this type of character ID in the in game chat.
Star Trek Online....same thing
Neverwinter Online....same thing
This sucks and needs to be changed or at the very least a way to HIDE your user ID. Displaying your user ID is a big problem and in a game like Elder Scrolls Online where symbols like @ are used when you are in RP is just ludicrous to say the least
However don't expect this to change its been that way in those other two games I just mentioned and the devs there just ignore any and all requests to change it.
The way I see it is this: The account wide aspect of characters is awesome. If you have a strong password, you will be fine. If you get hacked with a strong password, ESO will be obligated to help you get your account back and restored exactly as it should be. Problem solved, because the only problem is the user using abcdefg as a password.
That being said, I do feel for the RPers. That is something that should be easily fixed, along with guild chat, etc.