if they do enough damage to the leaves then it can affect how well the plant can photosynthesize (sp?)...it can stunt the growth of the plant.

I've heard of people having some success controlling Leaf Miners with Neem Oil.

They rarely do enough damage to harm the fruit and the chances are you will poison yourself by attempting to control them with chemicals. Try reducing the amount of fertilizer you use, that will make the plant less "sweet" to the miners and other pests, especially scale.