Everybody likes to feel useful. I think being useful is almost the same as being wanted or needed. To be told you're useless is akin to being told your not wanted or needed which can be hurtful. If you are describing someone as useless then try using some of the similes below to sugarcoat the hurt pill.
As useful as a third armpit
Useful as an ashtray on a motorbike
As useful as a glass door on a dunny
As useful as Anne Frank's drumkit
As useful as a hand knitted condom
As useful as an inflatable dartboard
He is like a lighthouse in the desert - extremely bright but bugger all use to anyone
As useful as a fart in a hurricane
As useful as a G rated porno film
As useful as a vacuum cleaner on the moon
As useful as forward gears on an Iraqi tank
As useful as a carpet fitters ladder
As useful as a concrete lifevest
As useful as a glass hammer
As useful as dinosaur repellent
As useful as a pedal powered wheelchair
As useful as a truckload of chihauhuas
