I totally understand and agree with all if the mentioned advantages of routing bathroom sink waste water to the black tank, and I have often wished mine were set up this way. But I can think of one potential disadvantage.
One of my kids must have left the sink running as we headed off to the playground, because when we came back there was very diluted grey water spilling out the door to our motorhome that overflowed up the shower drain. I shudder to think about what things would have looked like if it weren't just grey water. Now I make sure the water supply is shut off whenever we leave.