it is appropriate is the monopoly is caused by the natural factors (economies of scale).
Monopoly encourages research and development to take place. The abnormal profits achieved are used to fund research and development.
Monopoly encourages innovation because firm will want to join this market to also start enjoying the abnormal profits.
It makes it possible for production of some sensitive goods such goods are military equipments, supply of government services.
The economies of scale achieved may in the long run benefit the consumers in terms of superior products.