The thing is that this phenomenon (as you yourself hint) is very common, and I don't know if you could find an adjective equivalent to every PP to which it applies. (Actually it even applies to some NPs: I am cold and a long way from home). It seems that predicative phrases can often be co-ordinated without too much regard for their exact part of speech. Therefore in the case of co-ordinated predicates, co-ordination is not a safe test for part of speech. This was really the point I was making. The co-ordination test may suggest that "worth" is an adjective rather than a preposition, but it is not safe to say that it proves it.
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