I understand that screen_name is a kind of pseudo for username.
Well, if members want screen-name to appear on the screen instead of their own name, that’s great.S o when I list entries, I see the screen_name of the author.

But there is no tag parameter to list entries by screen_name !!!

All I can do is list entries by username with {exp:weblog:entries weblog="whatever" username="John Whoever"}.

I wish I have the same thing with screen_name, which is much more easy to remember.

etosha,

The reason, I believe, that this is not done that way is that the screen_name can be changed at any old time by the user on a whim.  Every 5 minutes or every day, every week or never and anything in between.

The username is a constant (or can be set to be a constant, unchangeable by the site admins) and that is why the tags look for that.  Otherwise one day you might have your pages quit displaying expected information simply because a user went and did something that they’re allowed to do.

Of course, tht doesn’t mean wanting to use the screen_name is a bad idea, indeed I can see reasons for wanting to do this and not caring if it suddenly is changed.  But it’s just the main reason I would think it is not a feature already. =)