From my understanding, long-term depression can be helpful as it can help reduce the synaptic strength of connections that are the result of maladaptive neuroplasticity. One example where long-term depression can be useful would be in patients with amputations who have phantom limb pain. PT can promote long-term depression of maladaptive pathways (causing the pain) via mirror therapy, which helps the body “relearn” normal sensations of the residual limb.
But to your point, long term depression can be negative too… I think “it depends” on the context (: