One of the reasons that many singers (floor singers rather than professional guests) "mumble" is that they will insist on singing from the words and sometimes even reading guitar chords at the same time. I have been known to use words myself, but only as an aide-memoire on the first time I sing a song in public. Even when I'm singing classical music the score is only really needed during rehearsal and thereafter usually only for entries. Most Musical Directors will request you to "Get those faces out of your copies !". To sing a song well and clearly you need to know it, otherwise you will never be able to get your timing and diction right. The most important thing is to WANT to communicate the song to your audience and not just to try and make a pretty sound.