Bah, lol. Maybe it simply delays evaluation then.
%a = 1
/alias %a <-- will evaluate %a to 1 and perform /alias 1
/inc %a <-- will not evaluate %a to 1 (so it doesn't try to increase 1), but will evaluate it later, so $(%a,0) becomes %a, and $null is nothing and so mIRC decides you didn't give a first parameter and echoes "insufficient parameters".
If that isn't right then I give up..
