I have no idea why it was added, only Khaled knows. The fact that it's shorter is probably the only reason for being added, and at the same time the reason for being depreciated. Since $r doesn't really indicate what the identifier does, in comparison with other built-in identifiers.
I looked in the full
versions.txt, and the first mention of $r is:
24/10/95 - mIRC v3.72
24.Added $r(v1,v2) identifier, returns random number
or letter between v1 and v2.
The first mention of $rand is:
08/01/99 - mIRC v5.5
124.Fixed freeze bug in $rand().
It's very doubtful that Khaled will have read this thread, though perhaps if he does he can give us some more info
Greets
