While that may look more like other mIRC scripting statements that take a code block, I think the use of ":" is a more suitable syntax. The primary reason is to show that the code following each case does not behave the same as a code block, ie. each case is not self-contained, but rather they cascade into one another.

Just to make it clear to anyone who might not know how switch/case statements (should) behave:
Code:
switch ($moo) {
  case 1:
    echo -a moo 1
  case 2:
    echo -a moo 2
    break
  case 3:
    echo -a moo 3
}


If $moo equals 1, that code would output the following:
Code:
moo 1
moo 2

Once a switch statement finds a case that matches, it executes all case statements following that one aswell until it reaches a break. The use of ":" rather than braces is a better indicator of that I think. Presumably this is also why other programming languages use the same structure.