Universal variables

[shawn wilson]

Variables such as %u, %r, etc are only interpreted sometimes. Is there
any reason they shouldn't always be expanded? Ie, if I want a config
that I could send to many users, I might want to do:
User %u123
Thinking that I can derrive a the username from what they are using to
login with. However, this doesn't work. So, I started wondering, It's
probably harder to limit where these variables are used vs just
allowing them universally, so why is this done?