they are a personal preference. If you want more movement in the tank, they will provide it. They also add more oxygen to the water, but they are not necessary because live plants do that as well.
As long as your tank has good circulation and some disruption of the water surface, air stones are not needed. Air bubbles are much more a decoration than a necessary ingredient.
Also depends partly on your selection of fish. Some are happier with more water movement in the tank (ie. butterfly loaches and other stream dwellers), others prefer stiller waters (ie. bettas)
i have zebra danios and that is it for now. and they like fast moving water. i bought two today so i will see if the danios look happeir in a day or so.