Tootler, frequently they do not make a good job of it, because they do not know the song well.
for a trained actor the situation is different he/she has acquired a reading and interpreting skill, most floorsingers reading from words do not have that skill,I have come across one or two exceptions.
is it acceptable for paid guests to have words? if it is not , then it could be argued that the same standard could be applied to floor singers.
another point against floor singers having words is that it creates a small barrier between them and the audience.
part of the skill of performing [in my opinion]is the abilty to ad lib if there is a temporary memory lapse