it just detracts from the look of the tank, that's ALL. it's all a matter of taste, i suppose. perhaps people like to think that their tanks are crystal clear or want to preserve an illusion that their inhabitants are floating. whatever the reason, bubbles won't harm anything - even if you put an airstone in the tank, itself. Pretty much the only thing that doesn't do well when exposed to air is a sponge. but you would have to take it out of the water for any damage to be done.