I think what BrnMix meant about harmful to plants is that if you saturate the water with air you can prevent CO2 from being absorbed into the water, and the plants need that to thrive and grow. I think that's the theory, anyway. Although since there is CO2 in the air as well, I'm not sure why an air stone wouldn't also put CO2 into the water. Probably oxygen just gets absorbed quicker than carbon dioxide.
Of course, for non CO2 injection tanks plants are recommended that either grow past the water surface or float on it, because contact with air enables them to get the necessary CO2.
Johnny Relentless, Sunrise, Florida
29G, with 12 hours of 65W light at 6700k, fluorite substrate, 12 ghost shrimp, a male Betta, and a female Betta, 6 Skirt Tetras, 3 Otos, 4 African Dwarf Frogs, a lot of Wisteria, some Ludwigia Repens, Java Fern, Java Moss, Riccia, Anacharis, Corckscrew Vals, Pennywort, Baby Tears, and Brazilian Sword Grass.