All things considered, your decision about mistletoe depends largely on your goals and the location of the infested tree. If your property is heavily wooded or you harbor birds and other wildlife, you may choose to let it grow and contribute to biodiversity in your area. If the tree is next to your house or driveway, a weakening limb may become a safety factor that requires removal. If the tree appears to be suffering adversely, you may want to remove the growth in order to improve the tree health.
Mistletoe can be removed by judicious pruning of the limb to remove the entire mistletoe root, which may be up to a foot long.