I've yet to hear a good reason why this would be very useful ("very" as in "useful enough to consume a part of Khaled's precious time in developing mirc"). The only use I can see is having shorter names for aliases. Meaning that this feature would allow single letter aliases of the same name, like this:
Code:
alias foo {
  ....
  alias -l i return $dll(blah.dll,Blah,$1)
  echo -a $i(one)
  echo -a $i(two)
}
alias bar {
  ....
  alias -l i return $dll(another.dll,Something,$1)
  echo -a $i(one)
  echo -a $i(two)
}
as opposed to the current way:
Code:
alias foo {
  ....
  echo -a $i1(one)
  echo -a $i1(two)
}
alias bar {
  ....
  echo -a $i2(one)
  echo -a $i2(two)
}
alias -l i1 return $dll(blah.dll,Blah,$1)
alias -l i2 return $dll(another.dll,Something,$1)

I don't even consider saving letters necessarily good because it could lead to less descriptive alias names (if the aliases foo and bar were huge and each of them used a bunch of $i()'s, don't you think that this could cause some confusion as to what $i() does?)

Do you think saving a few letters from alias names is "very" useful or do you have a better use in mind? Oh and before you mention something like "other languages support that", I don't find this argument strong enough. We're talking about mirc, which means we should take into consideration the nature of the mirc scripting language, the development status (one-man project) as well as the scripters base.