Moth Orchid

You're right, the roots provide stability as most orchids are epiphytes and don't grow in soil at all but they still perform the major function of providing water and nutrients to the plants. The leaves will perform this way as well but are only minor compared to the roots. The orchids growing on rocks are a type of lithophyte. Both types get most of their nutrients and water out of tiny particles in the air and sometimes from debris accumulating around it.

It's true, these are a type of air plant, known as an epiphyte. I don't know if I would go as far to say that they require little nutrients, light or water. Phalaenopsis do well with less light than most orchids but they still need water at least once a week and to be fertilized to thrive. The root system is not stunted. I've seen orchid roots burst out of their pot and totally cover it with roots, so that the pot they are growing in is not even visible any more!