Michael Durham Prince is a member of the US rock band Legs Diamond that formed in the mid 1970's, he plays keyboards and rhythm guitar, but is not a vocalist. The band still play occasionally and are due to release a new album sometime in the not too distant future. He is a studio recording engineer and has worked with artists including Beyonce, Usher and Chris Brown, as well as Michael Jackson.
I don't see how people have made such a mountain of this subject, the booklet clearly states that the album contains 9 previously unreleased vocal tracks performed by Michael Jackson. You'll notice the only time MJ's name is listed inside the booklet as 'vocals/backing vocals by' is when there is more than one person singing either lead or backing vocals, such as the track that has James Porte as a vocalist with Michael, or Lenny Kravitz is a backing vocalist. This is to show that Michael was working on his vocal, be it backing or lead, in collaboration with someone else, as they obviously wouldn't just be able to put "Vocals; James Porte" as that would not be correct, considering it is MJ singing, but with James. Michael Durham Prince is simply the vocal recordist on the tracks that he is listed on and they need to put his job to distinguish from other jobs that you can have in a recording studio. His specaility for the tracks was just to make sure that Michael's vocals were recorded properly and that was his job, he was obviously not involved in other parts of the recording or engineering of the tracks or he would have been listed as such.
To clear this up further, check some of your other CD album booklets, not necissarily ones by MJ and look at what it says about recording and engineering. I have a CD by say Peter Gabriel and it says, recorded by and then a bunch of names of people I've never heard of. It doesn't mean they played all the instruments and sang on it, they are the studio technicians who recorded the instruments and Peter's vocals. All the band members are listed individually with the instruments they play. And even in some instances this may not be the case, but it is still obvious that it is.