A lot of the tracks with incomplete vocals are a mixture of actual lyrics and "placeholders" (as in MJ is mumbling or scatting where the lyrics would be).
I think a lot of fans have felt frustrated at the amount of unfinished/possibly unusable material, but you also have to remember that Michael was incredibly prolific. I wouldn't say having sixty to seventy vocally complete tracks per album for any artist would be realistic. Also, given legal troubles, short film projects, tours, etc., he had other matters to handle in which his time was devoted to.