I suspect that it might seem open for a while, until the players started to get the sense that it didn't really matter what they interpreted the amorphous thing as being since the mechanical effects of losing the resource are always the same.
That's pretty much how hit points in general went for us. First it was the generic "you deal five damage". Then it was a range of imaginative "you slash this and dodge there and trip him and blood spurts out etc. etc.". Then it was "oh, what the hell, you deal five damage".
Some ambiguity is good (for example, it's fine that a Knowledge skill could refer to book learning or practical experience), but it's important that each mechanical element have some real-world meaning that could be concisely and cogently explained to a layman, and that the ambiguity doesn't go too far.