With partial private ownership of an SOE you run into the "a man can not serve two masters" problem. The aims of the private owners and the government are unlikely to be the same. Private investors will want to maximise profits while the government may well have political objectives it wants met. A firm can not do both and if it were to try it would just fail to achieve either.I suspect this is, however, a reason why the left opposes partial privatisation. It means that, unlike Michael Cullen's years as minister of finance, the government won't be able to use the power companies as a proxy tax by demanding higher and higher dividends.
More importantly, as I've pointed out here a number of times, listing the SOEs opens them up to the rules that apply to publicly listed companies. That is a very good thing.