Sentence:  In answer to this question I want to propose that "to" is in fact always <WSD>contentful</WSD>&mdash;that it is never mere Case marking, strictly speaking&mdash;but that in certain contexts (namely, in V's headed by Dative-Shifting verbs) its grammatical contribution effectively "reduces" to Case marking and therefore can be suppressed under Passive. 
Meaning of the word marked with <WSD> tags:  Having content.