Plant people hate mistletoe because it kills trees. Bird people, on the other hand, love it because it provides food and nesting sites for a great many species, many of them rare or threatened — examples include the endangered Regent Honeyeater, the threatened Diamond Firetail, and the rare Painted Honeyeater, which depends almost entirely on mistletoe and wanders all over the continent in search of it. And if mistletoe kills the odd tree, so much the better: standing dead timber provides nest hollow sites for all manner of birds, mammals and reptiles, and good feeding territory for phascogales, antechinuses, treecreepers and sitellas.
The problems occur when, because of land clearing, there are not enough trees to begin with. Isolated trees are prone to mistletoe attack (mistletoe likes the extra light) and are usually unhealthy — without natural shrubs and groundcover underneath the trees, many insect species cannot complete their life cycles. Leaf-eating and sap-sucking types no longer have a natural population of predatory insects to keep their numbers down, and the tree suffers. Throw in a dry year or two, and the mistletoe can be the last straw.