As a singer, I really don't mind if the KJ sings one song at the start of each rotation. After all, go to karaoke not only to sing, but to hear others sing (ok, not the really bad ones,
). In most cases I find that the KJ will drop out of the rotation when it gets busy.
There was one place I went to that had the KJ equipment on a mezzanine above the bar with a guy running that and the KJ/host was on the floor level to take slips and announce singers. Most of the time, they would also have a third person there for some reason (maybe just to operate the equipment while the other guy had his turn to sing). So that meant that each rotation would start with the three of them each singing a song... that was a bit irksome!