The question should have been "whom do you want to log for?" As napa182 stated, it's a safe bet to have a flood control. Last but not least, you need to make sure the person you are logging for exists:

on $*:text:/^!log/iS:#:{ 
  if !%x {
    inc -u4 %x 
    if !$2 {
      msg # Whom do you want a log for? 
    }
    else { 
      msg # $iif($strip($2) ison #,$timestamp $+(<,$2,>) $3-,$2 is not on # $+ !) 
    }
  }
}