The trouble with funny sayings is that they soon become hackneyed clichés and lose their edge. Like someone being as much use as a chocolate fireguard/teapot which was great when I first heard it but does really cut it anymore.
So we have to find replacements, like a one-legged man at an arse kicking contest or a condom machine in the Vatican or an ashtray in a motorcycle. And then they too start to wane which is why it’s good to come across a new one.
It is: as much use as (drum roll) a glass eye at a keyhole.
Actually, it was the title of a book by Mary Pettibone Poole. I’m not sure who she was, but she doesn’t half turn up a lot on quotations pages.