Sunlight is far far brighter than your fluorescent light setup. Seedlings which receive direct sunlight often enough and for long enough don't grow leggy. Even bright shade in a south-facing window is brighter than many fluorescent light systems. Lights are used where there is no suitable window space or where there is insufficient sunlight due to day length, clouds, or other shading. If you have the sunlight then use it, but pay attention because it is actually too bright for some seedlings.