Deadweight loss

When a monopoly seeks to maximise profit by producing at the level where its MR equal MC, it increases its producer surplus, but at the expense of consumer surplus.  

Not only is this unfair to consumers, there is a deadweight loss, which is a loss of economic efficiency.   This is created because the monopoly produce a quantity that does not maximise the sum of consumer surplus and producer surplus.  From the market perspective, monopoly is considered inefficient because this area where marginal benefit is still greater than marginal cost is not realised. 

See also: Consumer Surplus, Producer Surplus