Vagrant, you're right no good employer would ask someone what their faults are.
But a good interviewer will ask what someone considers could be an area for improvement, as it demonstrates how self aware a candidate is and that they are open to the fact they aren't superhuman.
Me for example, I do a demanding job, I'm classed as Senior Management (big fish but small pool, so not as grand as it migt sound lol!), and I'm pretty adept at a lot of things...but I'm lousy at formulas on excel?
I've been taught 4 times but I don't use it day in day out. So I might be a whizz with the basics, but once adding formulas comes into it, I'm cr*p. It took me a couple of stressful times before I admitted to myself it's not a strength, so I know to get help or delegate that one part of my role when it crops up.
If someone tells me there's nothing they could be better at then they are either not aware enough of their capabilities, or foolng themselves. I'm 47 and every day brings me something new to learn, about life, my job and myself.