Not sure how to respond to the first part of your question as it is something I see occasionally but certainly not the rule at the venues I attend. Being a singer, I sometimes close my eyes when I fear a distraction will remove me from the song I'm singing and I'm a firm believer in trying to respect the song.
Speaking the last line of a song always seemed to me to be self-depracating. Whether real or contrived could only be determined by more time spent evaluating the singer's character.
As for "some of the songs are v.e.r.y. long and quite boring", possibly the fault of the singer. However, our modern society may be responsible for the fact that the average listener has an attention span that doesn't allow anything in that requires too much time or effort.