I agree with you!

Karaoke can ruin our work.
Some time ago I also was thinking about maybe including a bit of Karaoke, and I'm glad I forgot about it before I even started it.

I did a wedding once and the father of the bride came to me asking if he could sing a song. I said yes and the man (I noticed later he was a bit drunk) wouldn't stop singing and talking and I couldn't mute the mic/get rid of him because he was the father of the bride!
Nowadays, I never do that unless it's a very special request, probably in a wedding, only as an example.

As Uncledave said, I also try to keep anybody away from my gear. If they are trying to read the lyrics or are very close to me, as soon as I finish the song I'm playing I invite the person(s) not to disturb my work and that always work. But all, of course, on a professional way. This can probably be a bit difficult for entertainers to do when they're still beginnig, but my advice is to do this or anyone could get the "gig" or "party" spoiled because of some "old man".

Luis Santos