Found this an interesting read on metaphors in interface design by Alan Cooper.
If we depend on finding metaphors to create user interfaces, we encounter the several minor problems already mentioned, but there are two more major problems: metaphors are hard to find and they constrict our thinking.
It may be easy to discover visual metaphors for physical objects like printers and documents. It can be difficult or impossible to find metaphors for processes, relationships, services and transformations, the most frequent use of software. It can be extremely daunting to find a useful visual metaphor for buying a ticket, changing channels, purchasing an item, finding a reference, setting a format, rotating a tool or changing resolution, yet these operations are precisely the type we find in software most frequently.
Don’t get me wrong, there is nothing bad about using a metaphor if one happens to fit the situation. Look! Here’s a twenty-dollar bill lying on the sidewalk. Of course I’ll pick it up; I’d be a fool not to! But then, I’d be a bigger fool if I decided to make my living finding misplaced twenty-dollar bills. Metaphors are like that: use ’em if you find ’em, but don’t bend your interface to fit some arbitrary metaphoric standard.