It is always wrong to believe on insufficient evidence because these kinds of poor practices of belief-formation turn us into careless, credulous believers.
William Kingdon Clifford did put it nicely: “No real belief, however trifling and fragmentary it may seem, is ever truly insignificant; it prepares us to receive more of its like, confirms those which resembled it before, and weakens others; and so gradually it lays a stealthy train in our inmost thoughts, which may someday explode into overt action, and leave its stamp upon our character.”
Careless believing turns us into easy prey for fake-news pedlars, conspiracy theorists and charlatans. And letting ourselves become hosts to these false beliefs is morally wrong because the error cost for society can be devastating.
Our beliefs influence our actions. And false beliefs about physical or social facts lead us into poor habits of action that in the most extreme cases could threaten our survival.