The shutdown option is here to let you decide to leave the computer running without your supervision because you need to leave it and you want it to finish its work. But when you start the script you often don't know how long will it take and that you will need to go sleep before it ends. You may even return to the computer again before it ends and may decide to continue using the computer without restart.
Yep, I've had sessions run all night and I've been sitting there the next morning when the shutdown warning has popped up and I've had to restart the machine.
Because of this, an ABORT button on the shutdown warning requester to abort the shutdown would be nice....
Didn't there used to be one?