Even if MJ would only sing one verse and then remain quiet for the rest of the song, I'd still prefer that over lip syncing. I can understand it a bit more for tv performances since they are supposed to be much more polished, but concerts are about live performance and connecting with an audience - and you simply cannot nearly do that as much when you lip sync.
I have often wondered what the Dangerous and HIStory tours would have been like if MJ focused a bit less on the increasing theatrics and a bit more on the music side of things. I know he was a perfectionist and dreamt big - wanting his shows to be a huge experience, which they were - but I think he underestimated the power and effect just his voice could have. And not just on us fans either: I remember sitting in the theatre watching This Is It, and noticing the two songs people seemed most affected by were Human Nature and I Just Can't Stop Loving You. Both featured little or relatively simple dancing and zero theatrics, but both highlighted his voice. Another example, what do you think would have made a bigger and more lasting impression on people: the spectacle with the tank, the soldier and the little girl on stage during Earth Song as seen in the HIStory Tour, or just MJ under a spotlight shouting his heart out ("tell me WHAT about it?!") as seen in the Royal Concert and at the WMAs. It's not even a debate, imo.
And if increased live singing would have come at the cost of a bit less dancing, I would have been totally fine with that - not every song needs insane choreography. MJ's vocals at times were not that great during the HIStory Tour, but I still enjoy watching performances of WBSS and the J5 medley much more than any of the lip-synced songs. That performance of Scream in Brunei where he sings one verse live is much more interesting than any of the others. It's also why I think the J5 set at MSG in 2001 was infinitely better than the solo set, and so on.