You're as useless as... insults. Not everyone's as useful as you so here's some ammunition to tell someone exactly how useless they are.
You're as useless as a gun without ammo.
You're as useless as a pen without ink.
You're as useless as a Computer without Internet.
You're as useless as a phone with 1% remaining battery life.
You're as useless as a red light at 3 AM.
You're as useless as a TV without Cable.
You're as useless as white-out to a computer data entry clerk.
You're as useless as Han Solo without Chewbacca and the Millennium Falcon.
You're as useless as an XBox without Llive.
You're as useless as 01100010 01101001 01101110 01100001 01110010 01111001 without 00110000
