The basic concerns with governments holding important industrial assets are protectionism, subsidy, and the corrosion of faith in the openness of markets.
http://www.economist.com/news/britain/2 ... ing-lossesThat's the general issue. If we keep the bank in government hands, does the government then have to involve itself in the running of the bank? How do we stop the bank getting political favours that are denied to its competitors? How do we know that it will never be used, for instance, to offer soft loans in order to keep politically sensitive factories open until just after an election?