Paul Krugman explains the con:
This was always, however, a case of bait-and-switch. Free markets, in which private businesses compete for customers, can accomplish great things, and are indeed the best way to organize most of the economy. But the case for free markets isn’t a case for private business where there is no market: There’s no reason to presume that private firms will do a better job when there isn’t any competition, because the government itself is the sole customer. In fact, studies of privatization often find that it ends up costing more than having government employees do the work.
Much more at the link.