The way I see it, it's a matter of limiting power. You could say the same of government:

What's the problem with authoritarian governments when you have a benevolent executive at the helm? According to Aristotle, this is actually the most effective of all governments. Of course, Aristotle is also quick to point out that this model is the most prone to corruption and can easily turn into a tyranny, the worst of all possible governments.

It is important to limit the amount of power that any organization has, preferably with internal and external controls. Our government, for instance is designed to be self-limiting through a series of checks and balances and prone to external pressures from the electorate.

Time has shown that "dollar voting" is effective, but only to a certain degree. Industry has proven that it can become so massive and powerful that it can simply overcome public opinion. When you can buy the competition, can't you also just use their name and act as though the consumer has a choice? Even with government restriction on trusts, that is an incredibly common practice today. Relying on consumers to be educated in the intricacies of the corporate structure for every purchase they make is a losing proposition.

Certainly, such a massive, all-encompassing organization could be a force for good, but what assurance is there that will always be so? And, more importantly, if such an entity were to operate at the expense of society, would there be a way to deny its ability to do so?

Secondly, corporate influence on the government is an indictment of corporate power more than the faults of government. If a burglar is able to get past a faulty security system, you don't blame the system for robbing you, remove it, and then act surprised when you get robbed more often. If corporations are having to influence government to bar entry to the market, how does removing this step help anyone? Giving them more direct ability to control the market is a step in the wrong direction.