What's good about mistletoe? If this parasitic plant infests your tree, like is happening to my neighbor's hackberry, it is probably hard to see any good.
But there is one good thing about mistletoe. It is the host plant (food source) for the caterpillars of the Great Purple Hairstreak butterfly.
These beautiful butterflies appear in my garden when the Frostweed and Fragrant Mistflower are in bloom. They are close to three times the size of a Gray Hairstreak butterfly and have an orange abdomen, red, white, and iridescent blue/purple markings. The insides of their wings are iridescent blue/purple. They keep their wings closed when they feed, but when they fly around my prairie, you can see the iridescent colors flash in the sunlight. Beauty sometimes comes from things you would least expect, like mistletoe and caterpillars.
Thanks to my neighbors for unknowingly providing a wildlife habitat for Great Purple Hairstreak caterpillars, but I hope the dying hackberry tree does not fall on your house.