Suppose I buy a TV that has been negligently designed, and it overheats and explodes and takes out both my eyes. Who do I sue? If a corporation is not a person, I can't sue Sony.
Your entire premise is flawed. "Only a person can hold legal liability" is a false statement. Nobody claims the government is a person, and yet you can sue the government.
An entity made up of individuals can hold legal liability without being entitled to Constitutional freedoms such as privacy.
That said, the context in which I brought this up is perhaps a little glib. I don't suppose I'm inherently opposed to the notion of corporate ownership, though I DO believe it needs to be weakened.
Treating a corporation as a person is an inherently unfair proposition, because a corporation inherently has more POWER than a single person. I hesitate to play constructionist, but the Constitution was CLEARLY designed to protect the rights of individuals, not of faceless abstract entities.
The notion that a corporation can drop hundreds of millions of dollars buying a candidate's way into Congress or the White House, and call it "free speech", is abhorrent. The mere definition of money as speech is a "some are more equal than others" proposition, and when you add corporate personhood, you effectively state that corporations are entitled to more free speech than actual human beings.
You've turned the issue on its head. You are suggesting that corporate personhood protects individuals from the abuses of corporations. In reality, it does the opposite. It doesn't grant the little guy a right to stand up to an unfeeling, powerful abstraction -- the little guy already has that right. It grants the unfeeling, powerful abstraction the right to claim that IT is in fact being oppressed by the little guy.
And in a dispute between two persons, one of whom has the resources of an actual person and the other of whom has the resources of a corporation -- well, it's pretty clear who the likely victor is. Especially if it's the same "person" who wrote the law and paid various politicians to sponsor and pass it.