Herb, I think it is useful to distinguish two separate sources of difficulty with lexical classes.  One, comes from the idea that the lexical classes are, as you put it so well, merely prototypes ("merely" is my addition).  So the personal pronouns are strongly pronouns displaying all their characteristics, while the indefinite pronouns are just barely pronouns.  They don't have antecedents, don't have case, gender, number, or person and they do form possessives with apostrophe s.  

But a second source of difficulty with lexical classes stems from the fact that words that fit centrally in one class can, in some contexts, function in an entirely different class.  We "book" reservations, build "stone" houses, and take a "drive."

Both of these characteristics of lexical classes seem to me to contribute to the difficulty we, and especially our students, have when we try to pin them down.

Peter Adams

Community College Baltimore County