Genie, I agree totally with what you say---(I'm a huge fan of harmony--except possibly on verses)----but it doesn't seem to indicate that it's a good idea for the leader to read his song directly off the page.   Which is the issue.

Orchestras and classical choruses have to pay attention to what's in the sheet music (though conductors are forever trying to get their choruses "out of the music"--i.e. watch the conductor).   A person leading a song however should not need a sheet--or book--to sing from.

So a parallel between a chorus or orchestra performance and a singaround is tenuous.   That's my only point.