We have never had sponges, I always use flannels and wash in machine on a very regular basis.
Kids went to stay with GP's who have bath sponges, kids love them and appear more likely to wash themselves (yey) if I have a sponge for each of them.
HOWEVER, can you wash a bath sponge in the machine? Will it disintegrate? Is it a health hazard - don't they just stay damp and harbour dirt????