I don't think it's as much the ear/sheet argument that applies here but the continuing dependence on the printed music.

Mind you, I cannot imagine a session group, other than (almost) complete beginners gathered around music stands playing such simple tunes but if that is what you do and are happy with I am not going to tell you are doing it wrong. I do think weaning yourself (or the group in this case) off the sheet will be beneficial when it comes to ear, memory and general musical development and will engender a freer approach to the tunes. More opportunities for growth in other words.