I'm with JF - much can be seen on a pre-flight, and much of it regularly isn't. I recall a while ago somebody flying an aircraft to me for an air test, insisting on doing my own pre-flight despite his having just landed it and finding a dozen or so serious faults.
Having said that, are pilots properly trained to see these faults? I had an interesting conversation with Whirlybird recently where she recounted being taught during her AHI course how to do an A-check, but was taught it by a LAME, not a pilot. That may arguably be a practice that should be spread.
And yes, before now I've flown somewhere for a meeting, walked back to my aircraft with another Engineer before departing, who has idly pointed out faults on my own aircraft that I'd missed on my pre-flight. Embarrassing but good sport. I know of a club where finding a fault on somebody else's private aircraft earns you a pint - it's in the club rules, and the standard of aircraft there is sparkling !
G