Some monopolies are good, some are bad.
Basic economics to explain why they're bad - loss of consumer freedom. There is no longer a variety of goods to chose from, at a variety of prices. If the monopoly has that large a market share, they can to a point control prices - undercut competitors to the point that they go out of business, then increase prices to above previous levels to make abnormal profits. The lack of alternative products means they can get away with providing goods of inferior quality as well.
Having said that, some monopolies are good/necessary - natural monopolies occur when it's more efficient for just one firm to be in the market. The example of this that I was taught was the British railway system. Network Rail owns the rail network in Britain, and thus gains large economies of scale when it comes to replacing tracks, or maintaining current tracks.
I guess another examples of this could be provision of water (regionally at least).
<Mizzle> ive been in a jailcel for the first time of my life during this night lol
<Mizzle> new experience
